Skip DirectoryIndex execution unless method is GET or POST, restoring

Skip DirectoryIndex execution unless method is GET or POST, restoring
2.2 behavior when using mod_dav. PR 54914.

Otherwise, variable behavior results: if no DirectoryIndex file is found,
mod_dav's r->handler runs as expected.  But if an index file is found,
r->handler will be changed by ap_internal_fast_redirect() to something
other than mod_dav's r->handler, while r->method is left unchanged,
usually leading to a 405 response.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1561447 13f79535-47bb-0310-9956-ffa450edef68