AGJS-201 Sinon.JS XMLHttpRequest#response workaround

not to be merged - just for discussion. rebase is needed and authentication unit tests should be improved according the test improvements @lholmquist made in AGJS-200 (reuse sinon.js server responses)

if you're fine with the workaround for Sinon.JS XMLHttpRequest#response, I'll update the PR so that the authentication unit tests are re-using the Sinon.JS server responses
'' responseType is same as 'text' and the response data type is String

@matzew the first commit already exists in master branch. but in order to merge it in 2.0 branch and make the unit tests pass we need a hack in SinonJS lib since it doesn't support the [XMLHttprequest#response attribute - W3C](http://www.w3.org/TR/XMLHttpRequest/#the-response-attribute) and unit tests are using the request.response. without the hack, request.response comes as undefined from SinonJS FakeXHR