BUG/MINOR: mux-fcgi: Don't url-decode the QUERY_STRING parameter anymore

BUG/MINOR: mux-fcgi: Don't url-decode the QUERY_STRING parameter anymore

In the CGI/1.1 specification, it is specified the QUERY_STRING must not be
url-decoded. However, this parameter is sent decoded because it is extracted
after the URI's path decoding. Now, the query-string is first extracted, then
the script part of the path is url-decoded. This way, the QUERY_STRING parameter
is no longer decoded.

This patch should fix the issue #769. It must be backported as far as 2.1.

(cherry picked from commit 0f17a4444e641b6e16bff49e3bd99466f637b272)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 327bbfc3b81a8c22e2bb645ee6f9bb43b70c0276)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>