test digest response is validated

Based on https://scrutinizer-ci.com/g/Ocramius/zf2/issues/master/files/library/Zend/Authentication/Adapter/Http.php?orderField=path&order=asc

I made some improvements to the existing code. A rewrite would be better...
The testcase alread has a group? I added a docblock to the test case.
Could you tell me to which group this specific test should belong?