mock_cognitoidp idtoken is missing AWS oauth headers
I am attempting to test a function that accepts a Cognito id token, fetches the public jwk for a cognito userpool, uses that key to decode and verify the token and extract the claims. It's based on sample code from an AWS tutorial. It works in a real environment with genuine Cognito id tokens, but I haven't yet gotten my moto unittests to work.

I have attempted to incorporate the test_token_verification example in test_cognitoidp.

The idtoken I am generating with my mock_cognito userpool and decoding with the provided jwks-public.json only has two headers: 

`{'alg': 'RS256', 'typ': 'JWT'}

In order to verify the cognito id token and extract claims, the code from AWS expects a 'kid' header, which appears in the jwks-public.json but is not in the headers.

From the token verification function:

> headers = jwt.get_unverified_headers(token)
        kid = headers['kid']
        

I seem to be encountering an issue related to the open PR [https://github.com/spulec/moto/pull/1689](https://github.com/spulec/moto/pull/1689)

I've attempted to implement this fix in my own unittest, building my own payload and headers and signing them with the dummy private key from this repo, but didn't make a lot of headway.


diff --git a/moto/cognitoidp/models.py b/moto/cognitoidp/models.py
index 93e297551..4b4e0a8b1 100644
--- a/moto/cognitoidp/models.py
+++ b/moto/cognitoidp/models.py
@@ -128,8 +128,12 @@ class CognitoIdpUserPool(BaseModel):
             "exp": now + expires_in,
         }
         payload.update(extra_data)
+        headers = {"kid": "dummy"}  # KID as present in jwks-public.json
 
-        return jws.sign(payload, self.json_web_key, algorithm="RS256"), expires_in
+        return (
+            jws.sign(payload, self.json_web_key, headers, algorithm="RS256"),
+            expires_in,
+        )
 
     def create_id_token(self, client_id, username):
         extra_data = self.get_user_extra_data_by_client_id(client_id, username)
diff --git a/moto/cognitoidp/urls.py b/moto/cognitoidp/urls.py
index 5d1dff1d0..09e675e70 100644
--- a/moto/cognitoidp/urls.py
+++ b/moto/cognitoidp/urls.py
@@ -5,5 +5,5 @@ url_bases = ["https?://cognito-idp.(.+).amazonaws.com"]
 
 url_paths = {
     "{0}/$": CognitoIdpResponse.dispatch,
-    "{0}/<user_pool_id>/.well-known/jwks.json$": CognitoIdpJsonWebKeyResponse().serve_json_web_key,
+    "{0}/(?P<user_pool_id>[^/]+)/.well-known/jwks.json$": CognitoIdpJsonWebKeyResponse().serve_json_web_key,
 }
diff --git a/tests/test_cognitoidp/test_cognitoidp.py b/tests/test_cognitoidp/test_cognitoidp.py
index 37e1a56a3..3b7037889 100644
--- a/tests/test_cognitoidp/test_cognitoidp.py
+++ b/tests/test_cognitoidp/test_cognitoidp.py
@@ -3,6 +3,7 @@ from __future__ import unicode_literals
 import json
 import os
 import random
+import requests
 import uuid
 
 import boto3
@@ -10,10 +11,10 @@ import boto3
 # noinspection PyUnresolvedReferences
 import sure  # noqa
 from botocore.exceptions import ClientError
-from jose import jws
+from jose import jws, jwk, jwt
 from nose.tools import assert_raises
 
-from moto import mock_cognitoidp
+from moto import mock_cognitoidp, settings
 from moto.core import ACCOUNT_ID
 
 
@@ -1309,3 +1310,80 @@ def test_admin_update_user_attributes():
             val.should.equal("Doe")
         elif attr["Name"] == "given_name":
             val.should.equal("Jane")
+
+
+# Test will retrieve public key from cognito.amazonaws.com/.well-known/jwks.json,
+# which isnt mocked in ServerMode
+if not settings.TEST_SERVER_MODE:
+
+    @mock_cognitoidp
+    def test_idtoken_contains_kid_header():
+        # https://github.com/spulec/moto/issues/3078
+        # Setup
+        cognito = boto3.client("cognito-idp", "us-west-2")
+        user_pool_id = cognito.create_user_pool(PoolName=str(uuid.uuid4()))["UserPool"][
+            "Id"
+        ]
+        client = cognito.create_user_pool_client(
+            UserPoolId=user_pool_id,
+            ExplicitAuthFlows=[
+                "ALLOW_ADMIN_USER_PASSWORD_AUTH",
+                "ALLOW_REFRESH_TOKEN_AUTH",
+                "ALLOW_ADMIN_NO_SRP_AUTH",
+            ],
+            AllowedOAuthFlows=["code", "implicit"],
+            ClientName=str(uuid.uuid4()),
+            CallbackURLs=["https://example.com"],
+        )
+        client_id = client["UserPoolClient"]["ClientId"]
+        username = str(uuid.uuid4())
+        temporary_password = "1TemporaryP@ssword"
+        cognito.admin_create_user(
+            UserPoolId=user_pool_id,
+            Username=username,
+            TemporaryPassword=temporary_password,
+        )
+        result = cognito.admin_initiate_auth(
+            UserPoolId=user_pool_id,
+            ClientId=client_id,
+            AuthFlow="ADMIN_NO_SRP_AUTH",
+            AuthParameters={"USERNAME": username, "PASSWORD": temporary_password},
+        )
+
+        # A newly created user is forced to set a new password
+        # This sets a new password and logs the user in (creates tokens)
+        password = "1F@kePassword"
+        result = cognito.respond_to_auth_challenge(
+            Session=result["Session"],
+            ClientId=client_id,
+            ChallengeName="NEW_PASSWORD_REQUIRED",
+            ChallengeResponses={"USERNAME": username, "NEW_PASSWORD": password},
+        )
+        #
+        id_token = result["AuthenticationResult"]["IdToken"]
+
+        # Verify the KID header is present in the token, and corresponds to the KID supplied by the public JWT
+        verify_kid_header(id_token)
+
+
+def verify_kid_header(token):
+    """Verifies the kid-header is corresponds with the public key"""
+    headers = jwt.get_unverified_headers(token)
+    kid = headers["kid"]
+
+    key_index = -1
+    keys = fetch_public_keys()
+    for i in range(len(keys)):
+        if kid == keys[i]["kid"]:
+            key_index = i
+            break
+    if key_index == -1:
+        raise Exception("Public key (kid) not found in jwks.json")
+
+
+def fetch_public_keys():
+    keys_url = "https://cognito-idp.{}.amazonaws.com/{}/.well-known/jwks.json".format(
+        "us-west-2", "someuserpoolid"
+    )
+    response = requests.get(keys_url).json()
+    return response["keys"]
['moto/cognitoidp/urls.py', 'moto/cognitoidp/models.py', 'tests/test_cognitoidp/test_cognitoidp.py']