BUG/MINOR: httpclient: use a placeholder value for Host header

A Host header must be present for http_update_host() to success.
htx_add_header(htx, ist("Host"), IST_NULL) was used but this is not a
good idea from a semantic point of view. It also tries to make a memcpy
with a len of 0, which is unrequired.

Use an ist("h") instead as a placeholder value.

This patch fixes bug #1439.
diff --git a/src/http_client.c b/src/http_client.c
index a07cf9e..35de71a 100644
--- a/src/http_client.c
+++ b/src/http_client.c
@@ -283,7 +283,7 @@
 	sl->info.req.meth = meth;
 
 	/* Add Host Header from URL */
-	if (!htx_add_header(htx, ist("Host"), IST_NULL))
+	if (!htx_add_header(htx, ist("Host"), ist("h")))
 		goto error;
 	if (!http_update_host(htx, sl, url))
 		goto error;