(CDAP-576) Headers for HttpResponse

This adds headers to ``HttpResponse``. 

The changes in the test case are mostly just defaulting the matching for headers to ``any``, and adding a specific test-case to test that headers are processed correctly.
It does:https://docs.oracle.com/javase/7/docs/api/java/net/URLConnection.html#getHeaderFields%28%29

It just doesn't return it as a ``Multimap``, but as a Map from String -> List<String> (which is the format I carried over here). But I think it'd be cleaner to convert to a MultiMap for our use.