If a buffered request body wasn't fully sent, such as on early upstream
response or limited by flow control, unsent buffers could remain in the
input or busy chains when switching to the next upstream server.  This
resulted either in the invalid request sent or a stalled connection.

The fix is to reset chains similar to ngx_http_upstream_reinit().

The module allows to use HTTP/2 protocol for proxying.
HTTP/2 proxying is enabled by specifying "proxy_http_version 2".

Example:

    server {
        listen 8000;

        location / {
            proxy_http_version 2;
            proxy_pass https://127.0.0.1:8443;
        }
    }

    server {
        listen 8443 ssl;
        http2 on;

        ssl_certificate certs/example.com.crt;
        ssl_certificate_key certs/example.com.key;

        location / {
            return 200 foo;
        }
    }