Fix duplication of the router parameter

The service parameter list had two router entries in it due to the
assumption that the parameter list never contained the router itself in
it.

server/core/service.cc

@@ -106,12 +106,12 @@ Service* service_alloc(const char* name, const char* router, MXS_CONFIG_PARAMETE
         dcb_enable_session_timeouts();
     }
 
-    // Store router, used when service is serialized
-    service_add_parameter(service, CN_ROUTER, router);
-
     // Store parameters in the service
     service_add_parameters(service, params);
 
+    // Store router, used when service is serialized
+    service_replace_parameter(service, CN_ROUTER, router);
+
     service->router_instance = router_api->createInstance(service, params);
 
     if (service->router_instance == NULL)