Wrong IAM get_user_policy() response
The result given back from get_user_policy() is wrong. 'PolicyDocument' contains the internal dict that is used by moto instead of the clean policy document.
```
import json
from moto import mock_iam
import boto3

iam = boto3.client('iam')
iam.create_user(UserName='testuser')
iam.put_user_policy(
    UserName='testuser',
    PolicyName='testpolicy',
    PolicyDocument=json.dumps({'key': 'value'}))
result = iam.get_user_policy(
    UserName='testuser', PolicyName='testpolicy')
print result
```

Results in:
```
{u'UserName': 'testuser', u'PolicyName': 'testpolicy', u'PolicyDocument': '\\n      {u\\'policy_document\\': u\\'{"key": "value"}\\', u\\'user_name\\': u\\'testuser\\', u\\'policy_name\\': u\\'testpolicy\\'}\\n      ', 'ResponseMetadata': {'RetryAttempts': 0, 'HTTPStatusCode': 200, 'RequestId': '7a62c49f-347e-4fc4-9331-6e8eEXAMPLE', 'HTTPHeaders': {u'Content-Type': u'text/plain', u'server': u'amazon.com'}}}
```
diff --git a/moto/iam/responses.py b/moto/iam/responses.py
index 9931cb8d0..786afab08 100644
--- a/moto/iam/responses.py
+++ b/moto/iam/responses.py
@@ -407,7 +407,7 @@ class IamResponse(BaseResponse):
         return template.render(
             user_name=user_name,
             policy_name=policy_name,
-            policy_document=policy_document
+            policy_document=policy_document.get('policy_document')
         )
 
     def list_user_policies(self):
['moto/iam/responses.py']