Allow empty headers in mod_proxy. See below:

Empty headers are allowed by RFC2068 section 4.2.  The presence or
absence of an empty header can be significant.  The current mod_proxy
of httpd 1.3.x removes empty headers. Change mod_proxy to preserve
empty headers.
Submitted by:	Christian von Roques <roques@mti.ag>
Reviewed by:	Graham Leggett, Chuck Murcko


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@88526 13f79535-47bb-0310-9956-ffa450edef68