Response: new http_content_type() method.

This new method allows to set the content_type response based in a
mimetype extension lookup againts the server mime's database. e.g:

   response->http_content_type(dr, "jpg");

the HTTP response will append the following to the response
HTTP headers:

   Content-Type: image/jpeg

Signed-off-by: Eduardo Silva <edsiper@gmail.com>