Adding support for setting nested iframes (i.e. as an array) to the WebDriverIO.

The application that I am testing makes heavy use of iframes, and overall WebDriverIO handles iframes better than Nightmare (though more on that later).

Summary

• Within block now accepts nested frames when using the WebDriverIO Helper. Previously only the Nightmare Helper was supported.
• Changed the default window size for the Helper tests because the tab tests were failing because the clickable element was "off screen"
• Fixed acceptance test cases which were failing in the Nightmare Helper because Nightmare has problems if the any further navigation (i.e. clicking on a link) is done from within an iframe.

Example

Here is the example Scenario taken out of the ./test/acceptance/within_test.js which shows the usage.

Scenario('within on nested iframe (depth=2) and mixed id and xpath selector @WebDriverIO', (I) => {
  I.amOnPage('/iframe_nested');
  within({frame: ['#wrapperId', '[name=content]']}, () => {
    I.fillField('rus', 'Updated');
    I.click('Sign in!');
    I.see('Email Address');
  });
  I.see('Nested Iframe test');
  I.dontSee('Email Address');
});

Tests

The following tests were run after the changes and everything looks good.

• WebDriverIO acceptance tests: node ./bin/codecept.js run -c ./test/acceptance/codecept.WebDriverIO.json --grep WebDriverIO
• Nightmare acceptance: node ./bin/codecept.js run -c ./test/acceptance/codecept.Nightmare.json --grep nightmare
• Core tests: npm tests
• WebDriverIO tests: mocha test/helper/WebDriverIO_test.js
• Nightmare tests: mocha test/helper/SeleniumWebdriver_test.js

Extra notes about iframes and Nightmare

The following works with WebDriverIO, but not with Nightmare. Nightware will say that it can not find the text Email Address, and it will print out the text of all elements from the iframe page before the I.click('Sign in!') was executed.

Scenario('within on iframe @WebDriverIO', (I) => {
  I.amOnPage('/iframe');
  within({frame: 'iframe'}, () => {
    I.fillField('rus', 'Updated');
    I.click('Sign in!');
    I.see('Email Address');
  });
  I.see('Iframe test');
  I.dontSee('Email Address');
});

I will chalk this up to a limitation with Nightmare and not due to my code changes since these tests were already failing when I made my fork.

P.S. This is my first pull request, so just let me know if I need to change something to get it into the 'standard' pull request format.