Fix :svn download errors with :latest casks

[apainintheneck]

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Note: This is an alternative solution to #13354.

This only computes the sha256 for version :latest casks that don't use the subversion download method. Currently with Casks there are three allowed download types.

1. Curl Get (default)
2. Curl Post (:post)
3. Subversion (:svn)

From what I can see both Curl download methods require single compressed files which can be hashed. From the documentation: URL to the .dmg/.zip/.tgz/.tbz2 file that contains the application.

That requirement apparently doesn't exist for Subversion downloads so I added a Cask#checksumable? method that checks if the url is using either of the Curl methods. This should still work with #13232 assuming that is given a different using directive.

Fixes #13359.

[bevanjkay]

I believe once #13232 we can change this to.
But until #13232 is merged this will result in an error.

@MikeMcQuaid Could we merge without support and include the changes in the other PR? I'm not sure on the status quo here.

Suggested change

	url.using.nil? || url.using == :post
	(url.using.nil? && url.only_paths.nil?) || url.using == :post

[MikeMcQuaid]

Let's just merge what will work for now assuming that #13232 isn't merged or at least gracefully handling it.

Suggested change

	url.using.nil? || url.using == :post
	url.using.blank? || url.using == :post

or

Suggested change

	url.using.nil? || url.using == :post
	(url.using.blank? && url.only_paths.blank?) || url.using == :post

[bevanjkay]

For now it'll need to be

Suggested change

	url.using.nil? || url.using == :post
	url.using.blank? || url.using == :post

[MikeMcQuaid]

Fine as-is if it'll work, otherwise pick one of the suggestions and then 👍🏻 to merge.

[MikeMcQuaid]

Let's just merge what will work for now assuming that #13232 isn't merged or at least gracefully handling it.

Suggested change

	url.using.nil? || url.using == :post
	url.using.blank? || url.using == :post

or

Suggested change

	url.using.nil? || url.using == :post
	(url.using.blank? && url.only_paths.blank?) || url.using == :post

[apainintheneck]

Okay, I updated url.using.nil? to url.using.blank?. Also, I'll just hop over to the sparse checkout branch and mention the change after this gets merged in.

[MikeMcQuaid]

Thanks again @apainintheneck!