User-agent must contain (gzip).

http://stackoverflow.com/questions/7140035/http-server-not-honoring-accept-encoding-gzip-unless-user-agent-is-known-browse

Reviewed in https://codereview.appspot.com/8288046/.
diff --git a/apiclient/model.py b/apiclient/model.py
index fda4ef2..9881a15 100644
--- a/apiclient/model.py
+++ b/apiclient/model.py
@@ -141,7 +141,7 @@
       headers['user-agent'] += ' '
     else:
       headers['user-agent'] = ''
-    headers['user-agent'] += 'google-api-python-client/' + __version__
+    headers['user-agent'] += 'google-api-python-client/%s (gzip)' % __version__
 
     if body_value is not None:
       headers['content-type'] = self.content_type