Yet another attempt to make mtls straight

Author: lidizheng

gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2

@@ -200,7 +200,7 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
                 raise ValueError('When providing a transport instance, '
                                  'provide its credentials directly.')
             self._transport = transport
-        elif isinstance(transport, str):
+        else:
             Transport = type(self).get_transport_class(transport)
             self._transport = Transport(
                 credentials=credentials,