Use cls parameter instead of class (#341)

Use cls parameter instead of explicit `Credentials` reference to allow subclassing

Author: emilecaron

packages/google-auth/google/oauth2/credentials.py

@@ -179,7 +179,7 @@ def from_authorized_user_info(cls, info, scopes=None):
                 'Authorized user info was not in the expected format, missing '
                 'fields {}.'.format(', '.join(missing)))
 
-        return Credentials(
+        return cls(
             None,  # No access token, must be refreshed.
             refresh_token=info['refresh_token'],
             token_uri=_GOOGLE_OAUTH2_TOKEN_ENDPOINT,