mod_headers: Allow % at the end of a Header value. PR 36609.

(Backport of r490156, r499567)

Reviewed by: niq, rpluem, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@539271 13f79535-47bb-0310-9956-ffa450edef68