[7.x] Added ability to simulate "withCredentials" in test requests

[kohlerdominik]

Hi all

I stumbled over a missing functionality in the test cases. As stated here, XMLHttpRequests allow a flag which is named "withCredentials" to include cookies in the requests.

The XMLHttpRequest.withCredentials property is a Boolean that indicates whether or not cross-site Access-Control requests should be made using credentials such as cookies, authorization headers or TLS client certificates. Setting withCredentials has no effect on same-site requests.

By default, all JSON-requests implicit use an empty cookies-array. But with the knowledge of this flag, in my opinion it makes sense to make it configurable whether json requests should include cookies or not.

This PR adds a method to test cases, which allow to use $this->withCredentials() in a test to make all json()-calls include the cookie-jars, which are available anyway.