Skip deferred expiry if the object is deleted

Calling `expire` on an Em.Resource instance only schedules the expiry
for the next run loop.

If at that time, the resource is destroyed, we
should skip changing `expireAt`, because it's an error to set properties
on destroyed objects.
Merge pull request #63 from zendesk/shajith/guard-expire

Skip deferred expiry if the object is deleted