BUG/MINOR: http: Set the response error state in http_sync_res_state

This is just typo. It may only report a wrong response message state in
"show errors" on the CLI.

This patch must be backported in 1.7.
diff --git a/src/proto_http.c b/src/proto_http.c
index 4386282..dfe76d1 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -5549,7 +5549,7 @@
 			goto http_msg_closed;
 		}
 		else if (chn->flags & CF_SHUTW) {
-			txn->req.err_state = txn->req.msg_state;
+			txn->rsp.err_state = txn->rsp.msg_state;
 			txn->rsp.msg_state = HTTP_MSG_ERROR;
 			s->be->be_counters.cli_aborts++;
 			if (objt_server(s->target))