core, mod_proxy: If a kept_body is present, it becomes safe for

subrequests to support message bodies. Make sure that safety
checks within the core and within the proxy are not triggered
when kept_body is present. This makes it possible to embed
proxied POST requests within mod_include.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@654968 13f79535-47bb-0310-9956-ffa450edef68