It's sometimes necessary to work with all the items in a Sparkle feed
to be able to correctly identify the newest version but livecheck's
`Sparkle` strategy only passes the `item` it views as newest into a
`strategy` block.
This updates the `Sparkle` strategy to optionally pass all items into
a `strategy` block, so we can manipulate them (e.g., filtering,
sorting). This is enabled by naming the first argument of the
strategy block `items` instead of `item`. `Sparkle` `strategy` blocks
where the first argument is `item` will continue to work as expected.
This necessarily updates `#item_from_content` (now
`items_from_content`) to return all items. I've decided to move the
sorting out of `#items_from_content`, so it simply returns the items
in the order they appear. If there is ever an exceptional situation
where we need the original order, this will technically allow for it.
The sorting has instead been moved into the `#versions_from_content`
method, to maintain the existing behavior. I thought about passing
the items into the `strategy` block in their original order but it
feels like sorting by default is the better approach for now (partly
from the perspective of maintaining existing behavior) and we can
always revisit this in the future if a cask ever requires the
original order.
Lastly, this expands the `Sparkle` tests to increase coverage. The
only untested parts are `#find_versions` (which currently
requires a network request) and a couple safeguard `raise` calls
when there's a `REXML::UndefinedNamespaceException` (which shouldn't
be encountered unless something is broken).
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