Applied some of Craig's suggestions.

Author: Orest Bolohan

oauth2client/client.py

@@ -629,14 +629,16 @@ def set_store(self, store):
     self.store = store
 
   def _expires_in(self):
-    """Get in how many seconds does the token expire."""
+    """Return the number of seconds until this token expires."""
     if self.token_expiry:
       now = datetime.datetime.utcnow()
       if self.token_expiry > now:
         time_delta = self.token_expiry - now
         return int(round(time_delta.days * 86400.0 +
                          time_delta.seconds +
                          time_delta.microseconds * 0.000001))
+      else:
+        return 0
 
   def _updateFromCredential(self, other):
     """Update this Credential from another instance."""