Proxy: use zero Content-Length while discarding request body.

Proxy: use zero Content-Length while discarding request body.

Previously, r->headers_in.content_length_n was used, which is not 0 till
the request body is fully discarded, and the request might hang.

Adapted for nginx: the same change is applied to the HTTP/2 proxy
module (proxy_http_version 2), which is not present in freenginx and
used r->headers_in.content_length_n in the same way.  With an HTTP/2
upstream, the stale length resulted in the request being rejected
by the upstream, since the stream was closed without the body.

Co-authored-by: Elijah Zupancic <e.zupancic@f5.com>
Signed-off-by: Elijah Zupancic <e.zupancic@f5.com>
Origin: <https://freenginx.org/hg/nginx/rev/51e0dc7137843feda75b497773f598d3836c60a1>