Merge r1622450 from trunk:

ab: increase request and response header size to 8192 bytes,
fix potential buffer-overflow in Server: header handling.

Submitted by: jkaluza
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1638069 13f79535-47bb-0310-9956-ffa450edef68