Modified $content_length to match available request body length. As long as the request body was discarded or there was an error during reading, it now follows r->headers_in.content_length_n and reflects the fact that no request body is available, similarly to how Content-Length as used by the proxy module does. This makes complex processing of various error pages safer, notably when using fastcgi_pass, uwsgi_pass, or grpc_pass, where the $content_length variable is used to set length.