Misc fixes in HTTP parsing

* HTTP headers are case insensitive (see RFC2616 4.2 for details)
* Check errors when parsing status line
* treat LWS around the colon in headers

Notes:
* i couldn't test it as i don't have a TS gateway (and no unitary test either), so feel free to test that for real
* we agree that http_response_recv() is still totally buggy