http: Support reading gzip/deflate compressed data

Derived from VLC's http module.
Original authors:
  Antoine Cellerier <dionoea@videolan.org>
  Sébastien Escudier <sebastien-devel@celeos.eu>
  Rémi Duraffort <ivoire@videolan.org>
  Rémi Denis-Courmont <remi@remlab.net>
  Francois Cartegnie <fcvlcdev@free.fr>

Normally, http servers shouldn't send this to us since we
don't advertise it with an Accept-Encoding header, but some
servers still do it anyway.

Signed-off-by: Martin Storsjö <martin@martin.st>