Can be used to properly test all headers, including the ones setting the return code, using xdebug_get_headers()

Main problems that were faced with previous tests of headers
- xdebug_get_headers doesn't return headers of type 'HTTP/1.1 return_code message'
- use of http_response_code was not tested as well

The trait can be improved if you think it's needed:
- take into account response code when set via the 3rd parameter of the 'header' function
- take into account headers with same name and assert on the final result

Traits can't be unit tested using the current phpunit version 3.7.37