BUG/MINOR: stats/htx: Call channel_add_input() when response headers are sent

This function will only increment the total amount of bytes read by a channel
because at this stage there is no fast forwarding. So the bug is pretty limited.

This patch must be backported to 1.9.
diff --git a/src/stats.c b/src/stats.c
index 26b1450..b393541 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -3136,6 +3136,7 @@
 	if (!htx_add_endof(htx, HTX_BLK_EOH))
 		goto full;
 
+	channel_add_input(&s->res, htx->data);
 	return 1;
 
   full:
@@ -3195,6 +3196,7 @@
 	if (!htx_add_endof(htx, HTX_BLK_EOH))
 		goto full;
 
+	channel_add_input(&s->res, htx->data);
 	return 1;
 
 full: