Fix possible TypeError on res.push when socket has been destroyed

This manifests when a connection is immediately severed or a user spams the refresh button.

If this happens and res.push has been called without a callback or in the form (url, headers, callback), callback() is incorrectly invoked, leading to a TypeError.
Merge pull request #98 from ssafejava/master

Fix possible TypeError on res.push when socket has been destroyed.