BUG/MINOR: http-ana: Set HTX_FL_PROXY_RESP flag if a server perform a redirect

It is important to not forget to specify the HTX resposne was internally
generated when a server perform a redirect. This information is used by the H1
multiplexer to choose the right connexion mode when the response is sent to the
client.

This patch must be backported to 2.1.
diff --git a/src/http_ana.c b/src/http_ana.c
index eac5944..a7e8abf 100644
--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -4254,6 +4254,8 @@
 	if (!htx_add_endof(htx, HTX_BLK_EOH) || !htx_add_endof(htx, HTX_BLK_EOM))
 		goto fail;
 
+	htx_to_buf(htx, &res->buf);
+	htx->flags |= HTX_FL_PROXY_RESP;
 	/*
 	 * Send the message
 	 */