Making `channel` information available in the `Item` is necessary
to be able to filter out unstable items using a `strategy` block. If
an item doesn't specify a channel, then it uses the default channel
(this is what Sparkle itself uses for updates). Channels like `beta`
are something we want to avoid for stable casks and this allows for
that type of [cask-specific] filtering.
It's technically possible to automatically filter out items that
aren't using the default channel (i.e., `channel != nil`) in
`#items_from_content` but some casks use an unstable version, so we
can't do this internally. That is to say, we wouldn't be able to
override internal filtering in a `strategy` block, as any omitted
items wouldn't be provided to the block. Conversely, if we pass all
items to a `strategy` block, we can easily filter by channel there.
We haven't been filtering by channel internally and we've only found
one cask where this has been a problem, so it seems fine for now.
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