Fix oauthlib update errors

It seems flask is providing a class which is kind of
hybrid between list & dict and provide this error
because of this. Providing a dict instead of the
werkzeug.datastructures.EnvironHeaders.

Closes-Bug: #1839393
Change-Id: Ic8e8e8153f4c812145ff3976c934c84f401f1019

keystone/api/os_oauth1.py

@@ -227,7 +227,7 @@ class AccessTokenResource(_OAuth1ResourceBase):
                 url,
                 http_method='POST',
                 body=flask.request.args,
-                headers=flask.request.headers)
+                headers=dict(flask.request.headers))
         except NotImplementedError:
             # Client key or request token validation failed, since keystone
             # does not yet support dummy client or dummy request token,

keystone/auth/plugins/oauth1.py

@@ -56,7 +56,7 @@ class OAuth(base.AuthMethodHandler):
             url,
             http_method='POST',
             body=flask.request.args,
-            headers=flask.request.headers,
+            headers=dict(flask.request.headers),
             realms=None
         )
         if not result: