Nightwatch: Basic auth with access_key not working

I created nightwatch.json file like below (with username and access_key properties):

"test_settings": {
        "default": {
            "launch_url": "http://localhost",
            "selenium_port": 4444,
            "selenium_host": "localhost",
            "silent": true,
            "cli_args": {
                "webdriver.firefox.profile": "testprofile"
            },
            "screenshots": {
                "enabled": false,
                "path": "screenshots/"
            },
            "desiredCapabilities": {
                "browserName": "firefox",
                "javascriptEnabled": true,
                "acceptSslCerts": true
            },
            "username": "examplelogin",
            "access_key": "password"
        }

but basic auth don't work and I get login window to fill my password and login after running tests. I looked at packets in fiddler and no authorization header was present. Any ideas what I'm doing wrong?

Nightwatch v0.9.6