ab: support custom HTTP method with -m argument.

Example: ab -n 1 -m PING http://127.0.0.1/

Submitted By: Roman Jurkov <winfinit gmail com>
Committed By: ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1601076 13f79535-47bb-0310-9956-ffa450edef68