mod_proxy: Fix a request corruption problem and a buffering problem

mod_proxy: Fix a request corruption problem and a buffering problem
which sometimes prevented proxy-sendchunks from working.

strlen() couldn't be used since no space had been allocated
for trailing NUL, so occasionally the T-E header field contained
garbage and a 400 error would be returned by the origin server.

The lack of a flush bucket after the final "0\r\n\r\n" was a
showstopper for my simple tests (reverse proxy to Apache 1.3 +
custom module which read the body).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@123727 13f79535-47bb-0310-9956-ffa450edef68