cask/cask: pass args when initalising methods

[bevanjkay]

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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.

[MikeMcQuaid]

Thanks @bevanjkay!