MINOR: httpclient/cli: change the User-Agent to "HAProxy"

Change the User-Agent from "HAProxy HTTP client" to "HAProxy" as the
previous name is not valid according to RFC 7231#5.5.3.

This patch fixes issue #1354.
diff --git a/src/http_client.c b/src/http_client.c
index ea97860..b949e09 100644
--- a/src/http_client.c
+++ b/src/http_client.c
@@ -45,7 +45,7 @@
  */
 
 static struct http_hdr default_httpclient_hdrs[2] = {
-		{ .n = IST("User-Agent"), .v = IST("HAProxy HTTP client") },
+		{ .n = IST("User-Agent"), .v = IST("HAProxy") },
 		{ .n = IST_NULL, .v = IST_NULL },
 };