BUG/MINOR: checks: Send the right amount of outgoing data for HTTP checks

HTTP health-checks now use HTX multiplexers. So it is important to really send
the amount of outgoing data for such checks because the HTX buffers appears
always full.

No backport needed.
diff --git a/src/checks.c b/src/checks.c
index 9b28295..c805c2b 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1983,13 +1983,15 @@
 		goto out;
 	};
 
-	if (conn->mux->snd_buf(cs, &check->bo, b_data(&check->bo), 0) <= 0) {
+
+	if (conn->mux->snd_buf(cs, &check->bo,
+			       (IS_HTX_CONN(conn) ? (htxbuf(&check->bo))->data: b_data(&check->bo)), 0) <= 0) {
 		if ((conn->flags & CO_FL_ERROR) || (cs->flags & CS_FL_ERROR)) {
 			ret = TCPCHK_EVAL_STOP;
 			goto out;
 		}
 	}
-	if (b_data(&check->bo)) {
+	if ((IS_HTX_CONN(conn) && !htx_is_empty(htxbuf(&check->bo))) || b_data(&check->bo)) {
 		cs->conn->mux->subscribe(cs, SUB_RETRY_SEND, &check->wait_list);
 		ret = TCPCHK_EVAL_WAIT;
 		goto out;
@@ -2385,12 +2387,13 @@
 	 *    TCPCHK_ACT_SEND. */
 	else if (check->current_step && check->current_step->action == TCPCHK_ACT_SEND) {
 		if (conn && b_data(&check->bo)) {
-			ret = conn->mux->snd_buf(cs, &check->bo, b_data(&check->bo), 0);
+			ret = conn->mux->snd_buf(cs, &check->bo,
+						 (IS_HTX_CONN(conn) ? (htxbuf(&check->bo))->data: b_data(&check->bo)), 0);
 			if (ret <= 0) {
 				if ((conn && conn->flags & CO_FL_ERROR) || (cs && cs->flags & CS_FL_ERROR))
 					goto out_end_tcpcheck;
 			}
-			if (b_data(&check->bo)) {
+			if ((IS_HTX_CONN(conn) && !htx_is_empty(htxbuf(&check->bo))) || b_data(&check->bo)) {
 				cs->conn->mux->subscribe(cs, SUB_RETRY_SEND, &check->wait_list);
 				goto out;
 			}