do use the XOAUTH2 url if configured

fix regression introduced by af640208e1.

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/Gmail.py

@@ -61,6 +61,8 @@ class GmailRepository(IMAPRepository):
         if url is None:
             # Nothing was configured, cache and return hardcoded one.
             self._oauth2_request_url = GmailRepository.OAUTH2_URL
+        else:
+            self._oauth2_request_url = url
         return self._oauth2_request_url
 
     def getport(self):