backport mod_proxy date parsing buffer over-read (CVE-2007-3847)

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


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@563329 13f79535-47bb-0310-9956-ffa450edef68