Added withCredentials property to fake xhr

Cross browser pattern to detect if CORS is supported is to create a XHR
object and see if “withCredentials” property is present. Without this
property, tests that use fake XHR will go through the code path asuming
that CORS is not supported.
Added test for checking if withCredentials is added to the fake xhr and set to default false (as specified by standards.)
now withCredentials property is added only when the xhr supports it.
Merge pull request #354 from gtothesquare/master

Added withCredentials property to fake xhr