Fix Management REST API port guard check

When deploying the Management REST API on the nxdev environment to qualify the 11.3, we discover that the guard checking the port doesn't work as expected when:

• Management REST API is configured to run behind the same port as regular REST API (ie: blank nuxeo.management.api.http.port)
• Nuxeo is behind a reverse proxy such as Nginx

The culprit code is:

    protected boolean requestIsOnConfiguredPort(ServletRequest request) {
        int port = request.getServerPort();
        String configPort = Framework.getProperty(MANAGEMENT_API_HTTP_PORT_PROPERTY,
                Framework.getProperty(PARAM_HTTP_PORT));
        return Integer.parseInt(configPort) == port;
    }

In the case presented above, port is 80.

We want to fix this guard.