mod_request: Insert the KEPT_BODY filter via the insert_filter

hook instead of during fixups. Add a safety check to ensure the
filters cannot be inserted more than once. [Graham Leggett,
Ruediger Pluem]


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@654952 13f79535-47bb-0310-9956-ffa450edef68