Force mod_proxy_fcgi to read the whole FCGI response

even when the content has not been modified (HTTP 304).

The problem is described in PR 59838. This patch should 
avoid bogus reads causing the following issues with
HTTP 304 responses:
- AH01068: Got bogus version X, expected 1
- AH01069: Got bogus rid X, expected 1
- AH01075: Error dispatching request to :
- HTTP 503 logged instead of 304 (even if the external 
  client gets correctly a 304)

As discussed on IRC the HTTP_PRECONDITION_FAILED use case
should be handled like the HTTP_NOT_MODIFIED one but it will
be done in a separate commit.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1752347 13f79535-47bb-0310-9956-ffa450edef68