[Enhancement] sendKeys should notify when you're passing in undefined

[MicahLC]

Is your feature request related to a problem? Please describe.

I am porting some tests to Nightwatch 2.0, and I had a test failing that was using the sendKeys API. I was passing in browser.Keys.DOWN_ARROW as part of an array of keys, when I needed to be passing in browser.Keys.ARROW_DOWN

Describe the solution you'd like

In sendKeys and other similar APIs, if the user is passing in an undefined value as part of the array arg (or on its own), the API should throw an error or somehow alert the caller that they're not passing in what they think they are.

Describe how the solution will work

Not sure on the details yet, don't have time to look at the source right now.

[AutomatedTester]

this might be better fixed in Selenium. Command is in https://github.com/SeleniumHQ/selenium/blob/trunk/javascript/node/selenium-webdriver/lib/webdriver.js#L2521-L2561