Fix "TypeError: unhashable type" when calling json_request

This bug was introduced as part of #43.  `dry_run` was appended to
construct_payload parameters but there was a single instance where
construct_payload was executed and relying on `is_json` parameter's
default value.

Author: dkong

gcm/gcm.py

@@ -271,7 +271,7 @@ def json_request(self, registration_ids, data=None, collapse_key=None,
         for attempt in range(retries):
             payload = self.construct_payload(
                 registration_ids, data, collapse_key,
-                delay_while_idle, time_to_live, dry_run
+                delay_while_idle, time_to_live, True, dry_run
             )
             response = self.make_request(payload, is_json=True)
             info = self.handle_json_response(response, registration_ids)