Facebook provider set incorrect token_type
I got following error, when I'm trying to get result.popup_html().
```
File "/home/user/.virtualenvs/kurjerzy-3.5.1/lib/python3.5/site-packages/authomatic/providers/oauth2.py", line 213, in to_tuple
    cls.TOKEN_TYPES.index(credentials.token_type))
ValueError: 'bearer' is not in list
```
credentials.token_type is "bearer" not "Bearer"

I think the fix is to add 

```
if data.get('token_type') == 'bearer':
    credentials.token_type = cls.BEARER
```
in oauth2.py, Facebook class, method _x_credentials_parser(credentials, data)
diff --git a/authomatic/providers/oauth2.py b/authomatic/providers/oauth2.py
index 696196b..7fe8777 100644
--- a/authomatic/providers/oauth2.py
+++ b/authomatic/providers/oauth2.py
@@ -859,7 +859,10 @@ class Facebook(OAuth2):
         
         # Facebook returns "expires" instead of "expires_in".
         credentials.expire_in = data.get('expires')
-        
+
+        if data.get('token_type') == 'bearer':
+            credentials.token_type = cls.BEARER
+
         return credentials
     
     
['authomatic/providers/oauth2.py']