Headers missing for requests passing though fakeServer

[revathskumar]

When using interceptor on pages which using cookie session, the non intercepted requests are not sending with cookies which causes request to respond back with 401.

Nise, won't allow to setHeader for each request when using the fakeServer. We need a xhr.onCreate callback to be exposed by fakeServer for this ability. ALso sinon has black listed headers as per the spec which includes Cookie.

I have raise a issue on sinonjs/nise#49