Merge r1752347 from trunk:

Merge r1752347 from trunk:

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.

Submitted by: elukey
Reviewed/backported by: jim

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1757670 13f79535-47bb-0310-9956-ffa450edef68