BUG/MINOR: http: http-request replace-path duplicates the query string

BUG/MINOR: http: http-request replace-path duplicates the query string

In http_action_replace_uri() we call http_get_path() in the case of
a replace-path rule. http_get_path() will return an ist pointing to
the start of the path, but uri.ptr + uri.len points to the end of the
uri. As as result, we are matching against a string containing the
query, which we append to the "path" later, effectively duplicating
the query string.

This patch uses the iststop() function introduced in "MINOR: ist: add
an iststop() function" to find the '?' character and update the ist
length when needed.

This fixes issue #510.

The bug was introduced by commit 262c3f1a ("MINOR: http: add a new
"replace-path" action"), which was backported to 2.1 and 2.0.