Auto-respond to successive requests made with a single XHR object

Fix fakeServer not auto-responding in case a timeout elapses between creating a
XHR object and sending a request with it. This XHR usage pattern occurs in code
which caches a single XHR instance and does several requests with it.
Merge pull request #349 from jansuchy/auto-respond-successive-xhr

Auto-respond to successive requests made with a single XHR object