mod_proxy_uwsgi: Error out on HTTP header larger than 16K

The uwsgi protocol does not let us serialize more than 16K of HTTP header,
so fail early with 500 if it happens.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1880205 13f79535-47bb-0310-9956-ffa450edef68
