Skip message-body with HTTP 304 responses and mod_proxy_fcgi with no conn reuse.

Background: https://bz.apache.org/bugzilla/show_bug.cgi?id=59838

This commit should be an improvement for http://svn.apache.org/r1752347, in which
I tried to resolve a long standing issue with mod_proxy_fcgi: when a FCGI backend
returns a response that will become a HTTP 304, mod_proxy_fcgi forces a break
in the request processing ending up in reading the remaining bytes (for example
the message-body) and trying to intepret them as a FCGI header. This would cause
a error and a weird behavior:
1) HTTP 304 response flushed to the client correctly.
2) HTTP 503 logged in the access_log plus other bogus errors in the error_log.
I tried to separate the behavior when connection reuse is set or not, to eliminate
unnecessary latency in the latter case (which is the default).

I tested the change with and without connection reuse successfully, but feedback is
really welcome. If nobody will have anything against it I'll then proceed to update
my 2.4.x backport proposal.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1754732 13f79535-47bb-0310-9956-ffa450edef68