Add `google.oauth2.service_account.IDTokenCredentials`. (#234)

diff --git a/google/auth/app_engine.py b/google/auth/app_engine.py
index fa13f8e..f47dae1 100644
--- a/google/auth/app_engine.py
+++ b/google/auth/app_engine.py
@@ -136,7 +136,7 @@
 
     @_helpers.copy_docstring(credentials.Scoped)
     def with_scopes(self, scopes):
-        return Credentials(
+        return self.__class__(
             scopes=scopes, service_account_id=self._service_account_id)
 
     @_helpers.copy_docstring(credentials.Signing)