Update gcm.py

Fixed a situation where if the TTL was 0 the if statement would effectively fail.

A TTL of zero means deliver now or never which is necessary for my use case.

Author: NetworksAreMadeOfString

gcm/gcm.py

@@ -102,7 +102,7 @@ def construct_payload(self, registration_ids, data=None, collapse_key=None,
         if delay_while_idle:
             payload['delay_while_idle'] = delay_while_idle
 
-        if time_to_live:
+        if time_to_live >= 0:
             payload['time_to_live'] = time_to_live
             if collapse_key is None:
                 raise GCMNoCollapseKeyException("collapse_key is required when time_to_live is provided")