mod_proxy_ajp: Support unknown HTTP methods.

mod_proxy_ajp: Support unknown HTTP methods.
BZ 54416

Forward unknown methods as request attributes
using method id SC_M_JK_STORED=0xFF and
request attribute id SC_A_STORED_METHOD=13.

Code ported from mod_jk (which got it from mod_jk2).

Supported by Tomcat at least since TC 4.1.
Doesn't seem to be supported by Jetty though.

Backport of r1435178 from trunk resp. r1436400
from 2.4.x.

Submitted/backported by: rjung
Reviewed by: wrowe, rpluem

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1446624 13f79535-47bb-0310-9956-ffa450edef68