backport mod_proxy date parsing buffer over-read

Submitted by: Nick Kew, Davi Arnaut
Reviewed by: niq, rpluem, covener


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@563198 13f79535-47bb-0310-9956-ffa450edef68