mod_proxy_ajp: Fix get_content_length().

clength in request_rec is for response sizes,
not request body size. It is initialized to 0,
so the "if" branch was never taken.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1649043 13f79535-47bb-0310-9956-ffa450edef68