introduced FakeResponseWriter and FakeDispatcher for testing purposes
removed safehttp.ResponseRecorder
removed the dreaded safehttp.DeprecatedNewResponseWriter (!)
removed change detectors in Interceptor tests (e.g. when headers were
checked for ones that are introduced directly by the Dispatcher
implementation, not the Interceptor itself)
Complete overhaul of the safehttptest package.
Fixes #152 (obsolete).