Fix mock doc contains method

[erwinheitzman]

Fix for #3357

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Unit tests (npm test) were run locally and passed
• E2E Tests (npm run test.karma.prod) were run locally and passed
• Prettier (npm run prettier) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

GitHub Issue Number: #3357

What is the new behavior?

Now follows the spec as found here https://developer.mozilla.org/en-US/docs/Web/API/Node/contains

Does this introduce a breaking change?

• Yes
• No

Testing

I have added all the scenarios describes in the spec.

[rwaskiewicz]

Looks good to me - thanks!