Webform variants not respected for default values

1.0 Problem/Motivation

While Webform variants largely work, `fillForm()`, when setting default values, doesn't account for them.

2.0 Detailed steps to reproduce (embed screenshots)

* Create a Webform with Civi integration enabled.
* Set a default value on a form element.
* Create a variant with a different value for the same form element.
* View the variant.

The default value will be the value of the non-variant version of the form.

3.0 Proposed resolution

I have a PR, will submit it for CiviCARROT to find obvious issues, then do more manual testing.