If we are going to get the length of a brigade, it helps if that

brigade has data.  To that end, if we have just expanded ctx->b, we need
to concat ctx->b to the end of b, so that b has something to pass
back to the previous filter.

This fixes the problem with the proxy not proxying non-keepalive
connections.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90046 13f79535-47bb-0310-9956-ffa450edef68