Trap missing response/headers in getAllResponseHeaders (fix for #27)

[jjbenson]

This is a work-around for issue #27 (Sync requests via jQuery fail).

[oparisy]

I encountered issues #27 and #47 while using node-XMLHttpRequest to mock xhr while testing a module (of which code I have no control of).

But everything works properly if I make a modification similar to this pull request.

Is it considered for inclusion in a future release?