When a media file contains valid UTF-8, git-remote-mediawiki tried to be
too clever about the encoding, and the call to utf8::downgrade() on the
downloaded content was failing with

  Wide character in subroutine entry at git-remote-mediawiki line 583.

Instead, use $response->decode() to apply decoding linked to the
Content-Encoding: header, and return the content without attempting any
charset decoding.
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>