Support authentication using Authorization header

According to RFC 6749, section 2.3.1:

> Including the client credentials in the request-body using the two
> parameters is NOT RECOMMENDED and SHOULD be limited to clients unable
> to directly utilize the HTTP Basic authentication scheme (or other
> password-based HTTP authentication schemes).

This changeset adds an optional parameter, `authorization_header` that
makes it easier to use this client library with OAuth2 providers that
support client authentication via the HTTP Authorization header
instead of in the request body.

oauth2client/client.py

@@ -1784,7 +1784,9 @@ class OAuth2WebServerFlow(Flow):
   """
 
   @util.positional(4)
-  def __init__(self, client_id, client_secret, scope,
+  def __init__(self, client_id,
+               client_secret=None,
+               scope=None,
                redirect_uri=None,
                user_agent=None,
                auth_uri=GOOGLE_AUTH_URI,
@@ -1792,6 +1794,7 @@ class OAuth2WebServerFlow(Flow):
                revoke_uri=GOOGLE_REVOKE_URI,
                login_hint=None,
                device_uri=GOOGLE_DEVICE_URI,
+               authorization_header=None,
                **kwargs):
     """Constructor for OAuth2WebServerFlow.
 
@@ -1819,9 +1822,14 @@ class OAuth2WebServerFlow(Flow):
         proper multi-login session, thereby simplifying the login flow.
       device_uri: string, URI for device authorization endpoint. For convenience
         defaults to Google's endpoints but any OAuth 2.0 provider can be used.
+      authorization_header: string, For use with OAuth 2.0 providers that
+        require a client to authenticate using a header value instead of passing
+        client_secret in the POST body.
       **kwargs: dict, The keyword arguments are all optional and required
                         parameters for the OAuth calls.
     """
+    if scope is None:
+      raise TypeError("The value of scope must not be None")
     self.client_id = client_id
     self.client_secret = client_secret
     self.scope = util.scopes_to_string(scope)
@@ -1832,6 +1840,7 @@ class OAuth2WebServerFlow(Flow):
     self.token_uri = token_uri
     self.revoke_uri = revoke_uri
     self.device_uri = device_uri
+    self.authorization_header = authorization_header
     self.params = {
         'access_type': 'offline',
         'response_type': 'code',
@@ -1957,10 +1966,11 @@ class OAuth2WebServerFlow(Flow):
 
     post_data = {
         'client_id': self.client_id,
-        'client_secret': self.client_secret,
         'code': code,
         'scope': self.scope,
     }
+    if self.client_secret is not None:
+      post_data['client_secret'] = self.client_secret
     if device_flow_info is not None:
       post_data['grant_type'] = 'http://oauth.net/grant_type/device/1.0'
     else:
@@ -1970,7 +1980,8 @@ class OAuth2WebServerFlow(Flow):
     headers = {
         'content-type': 'application/x-www-form-urlencoded',
     }
-
+    if self.authorization_header is not None:
+      headers['Authorization'] = self.authorization_header
     if self.user_agent is not None:
       headers['user-agent'] = self.user_agent
 

tests/test_oauth2client.py

@@ -1027,6 +1027,29 @@ class OAuth2WebServerFlowTest(unittest.TestCase):
     request_code = urllib.parse.parse_qs(http.requests[0]['body'])['code'][0]
     self.assertEqual(code, request_code)
 
+  def test_exchange_using_authorization_header(self):
+    auth_header = 'Basic Y2xpZW50X2lkKzE6c2VjcmV0KzE=',
+    flow = OAuth2WebServerFlow(
+      client_id='client_id+1',
+      authorization_header=auth_header,
+      scope='foo',
+      redirect_uri=OOB_CALLBACK_URN,
+      user_agent='unittest-sample/1.0',
+      revoke_uri='dummy_revoke_uri',
+        )
+    http = HttpMockSequence([
+      ({'status': '200'}, b'access_token=SlAV32hkKG'),
+    ])
+
+    credentials = flow.step2_exchange('some random code', http=http)
+    self.assertEqual('SlAV32hkKG', credentials.access_token)
+
+    test_request = http.requests[0]
+    # Did we pass the Authorization header?
+    self.assertEqual(test_request['headers']['Authorization'], auth_header)
+    # Did we omit client_secret from POST body?
+    self.assertNotIn('client_secret', test_request['body'])
+
   def test_urlencoded_exchange_success(self):
     http = HttpMockSequence([
       ({'status': '200'}, b'access_token=SlAV32hkKG&expires_in=3600'),