[MINOR] http redirect: use proper call to return last response

During a redirect, we used to send the last chunk of response with
stream_int_cond_close(). But this is wrong in case of pipeline,
because if the response already contains something, this function
will refrain from touching the buffer. Use a concatenation function
instead.

Also, this call might still fail when the buffer is full, we need
a second fix to refrain from parsing an HTTP request as long as the
response buffer is full, otherwise we may not even be able to return
a pending redirect or an error code.

src/proto_http.c

@@ -2855,7 +2855,8 @@ int http_process_req_common(struct session *s, struct buffer *req, int an_bit, s
 				/* keep-alive not possible */
 				memcpy(rdr.str + rdr.len, "\r\nConnection: close\r\n\r\n", 23);
 				rdr.len += 23;
-				stream_int_cond_close(req->prod, &rdr);
+				buffer_write(req->prod->ob, rdr.str, rdr.len);
+				stream_int_cond_close(req->prod, NULL);
 				goto return_prx_cond;
 			}
 		}