MINOR: http-htx: Add an option to eval query-string when the path is replaced

MINOR: http-htx: Add an option to eval query-string when the path is replaced

The http_replace_req_path() function now takes a third argument to evaluate the
query-string as part of the path or to preserve it. If <with_qs> is set, the
query-string is replaced with the path. Otherwise, only the path is replaced.

This patch is mandatory to fix issue #829. The next commit depends on it. So be
carefull during backports.