cask/cask: pass args when initalising methods

[bevanjkay]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

When a custom url is passed to the ExtractPlist livecheck strategy, we were creating a copy of the cask and replacing the url. This was being parsed as a block, which was causing a deprecation error now that url do blocks are deprecated.

This PR instead creates a minimal copy of the cask and passes it to CaskLoader as content instead of copying the entire cask.

[bevanjkay]

I had added a test so that we can catch this regression in future, as I don't think it's the first time we've had some issues here.