Picklive/Sinon.JS — comment on commit 6f0232553a6ef4d8df1a7ec455a31f4f73972070
File: lib/sinon/util/fake_xml_http_request.js line 115
By timruffles:

The FakeXMLHttpRequest constructor specify which URL is going to be requested. It's only when you call XHR.open that you know whether to defake or not.