BUG/MINOR: http_htx: Initialize HTX error messages for TCP proxies

Since the HTX is the default mode for all proxies, HTTP and TCP, we must
initialize all HTX error messages for all HTX-aware proxies and not only for
HTTP ones. It is required to support HTTP upgrade for TCP proxies.

This patch must be backported to 2.0.
diff --git a/src/http_htx.c b/src/http_htx.c
index cd19d8f..0e788c1 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -680,7 +680,7 @@
 	int err_code = 0;
 
 	for (px = proxies_list; px; px = px->next) {
-		if (px->mode != PR_MODE_HTTP || !(px->options2 & PR_O2_USE_HTX))
+		if (!(px->options2 & PR_O2_USE_HTX))
 			continue;
 
 		for (rc = 0; rc < HTTP_ERR_SIZE; rc++) {