[google-analytics] Google Analytics Integration extractCheckoutOptions bug, Impact Checkout Step Viewed and Completed

Migrated from #36 by @nickrathwell

Base on the documentation:

https://segment.com/docs/spec/ecommerce/v2/#checkout-step-viewed
https://segment.com/docs/spec/ecommerce/v2/#checkout-step-completed

The checkout options are to be returned in this format:

analytics.track('Checkout Step Completed', {
  checkout_id: '50314b8e9bcf000000000000',
  step: 2,
  shipping_method: 'Fedex',
  payment_method: 'Visa'
});

However the Google Analytics Integration is coded to look for these value in that format

analytics.track('Checkout Step Completed', {
  checkout_id: '50314b8e9bcf000000000000',
  step: 2,
  shipping_method: 'Fedex',
  payment_method: 'Visa'
});

See code https://github.com/segment-integrations/analytics.js-integration-google-analytics/blob/master/lib/index.js#L903

function extractCheckoutOptions(props) {
  var options = [
    props.paymentMethod,
    props.shippingMethod
  ];
  // Remove all nulls, and join with commas.
  var valid = reject(options);
  return valid.length > 0 ? valid.join(', ') : null;
}

Start in lib/index.js around extractCheckoutOptions at the referenced line, then trace how checkout step events pass their properties into the integration. Compare the documented checkout payload with the property names the function reads; done means Checkout Step Viewed and Checkout Step Completed produce the intended checkout options.