Allow headers to be overriden on oauth2.fetch_token

Twitters application-only client credentials flow
struggle when provided with an accept header. This
allow users to omit it. Also include a new default
encoding and charset content type header.

requests_oauthlib/oauth2_session.py

@@ -112,7 +112,7 @@ def authorization_url(self, url, state=None, **kwargs):
 
     def fetch_token(self, token_url, code=None, authorization_response=None,
             body='', auth=None, username=None, password=None, method='POST',
-            verify=True, **kwargs):
+            headers=None, verify=True, **kwargs):
         """Generic method for fetching an access token from the token endpoint.
 
         If you are using the MobileApplicationClient you will want to use
@@ -131,6 +131,7 @@ def fetch_token(self, token_url, code=None, authorization_response=None,
         :param method: The HTTP method used to make the request. Defaults
                        to POST, but may also be GET. Other methods should
                        be added as needed.
+        :param headers: Dict to default request headers with.
         :param verify: Verify SSL certificate.
         :param kwargs: Extra parameters to include in the token request.
         :return: A token dict
@@ -153,22 +154,28 @@ def fetch_token(self, token_url, code=None, authorization_response=None,
                 redirect_uri=self.redirect_uri, username=username,
                 password=password, **kwargs)
 
+        headers = headers or {
+            'Accept': 'application/json',
+            'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8',
+        }
         if method.upper() == 'POST':
             r = self.post(token_url, data=dict(urldecode(body)),
-                headers={'Accept': 'application/json'}, auth=auth,
+                headers=headers, auth=auth,
                 verify=verify)
             log.debug('Prepared fetch token request body %s', body)
         elif method.upper() == 'GET':
             # if method is not 'POST', switch body to querystring and GET
             r = self.get(token_url, params=dict(urldecode(body)),
-                headers={'Accept': 'application/json'}, auth=auth,
+                headers=headers, auth=auth,
                 verify=verify)
             log.debug('Prepared fetch token request querystring %s', body)
         else:
             raise ValueError('The method kwarg must be POST or GET.')
 
         log.debug('Request to fetch token completed with status %s.',
                   r.status_code)
+        log.debug('Request headers were %s', r.request.headers)
+        log.debug('Request body was %s', r.request.body)
         log.debug('Response headers were %s and content %s.',
                   r.headers, r.text)
         log.debug('Invoking %d token response hooks.',

tests/test_compliance_fixes.py

@@ -19,6 +19,7 @@ def test_fetch_access_token(self):
         facebook.post = mock.MagicMock()
         response = requests.Response()
         response.status_code = 200
+        response.request = mock.MagicMock()
         response._content = 'access_token=urlencoded'.encode('UTF-8')
         response.headers['Content-Type'] = 'text/plain'
         facebook.post.return_value = response
@@ -38,6 +39,7 @@ def test_fetch_access_token(self):
         linkedin.post = mock.MagicMock()
         response = requests.Response()
         response.status_code = 200
+        response.request = mock.MagicMock()
         response._content = '{"access_token":"linkedin"}'.encode('UTF-8')
         linkedin.post.return_value = response
 
@@ -56,6 +58,7 @@ def test_fetch_access_token(self):
         weibo.post = mock.MagicMock()
         response = requests.Response()
         response.status_code = 200
+        response.request = mock.MagicMock()
         response._content = '{"access_token":"weibo"}'.encode('UTF-8')
         weibo.post.return_value = response