Update HttpTimeoutTest to expect 504 instead of 500 status code (#2958)

Author: christiangoerdes

core/src/test/java/com/predic8/membrane/core/transport/http/HttpTimeoutTest.java

@@ -106,7 +106,7 @@ void httpTimeout() throws Exception {
             Exchange exc = get("http://localhost:3023").buildExchange();
             client.call(exc);
 
-            assertEquals(500, exc.getResponse().getStatusCode());
+            assertEquals(504, exc.getResponse().getStatusCode());
         }
 
         watch.stop();