The following code is not correctly working:

@GET
public Object get() {
return Response.status(500).build();
}

because the returned ResponseImpl instance will not be handled like a javax.ws.rs.core.Response object (although it is derived from it)
The error is in ResourceMethod.invokeOnTarget() method:

if (method.getReturnType().equals(Response.class))

This can be fixed by using isAssignableFrom() instead of comparing classes by equality.