Allow IN with subselect to be preparable

[bensheldon]

Motivation / Background

This Pull Request has been created because I noticed that all SQL IN statements get automatically marked as non-preparable (Prepared Statements). But it seems like subselects should be preparable, at least to the extent that their Arel nodes are also preparable. With this change, the list of non-preparable Arel Nodes becomes:

• SqlLiteral, because they can contain anything.
• In when passed Arrays, because Arrays can contain an arbitrary number of items, as well as literal values and thus can have high variability and are therefore bad candidates for a Prepared Statement.
• NotIn when passed Arrays, for the same reason as In
• HomogenousIn which can only be passed Arrays

Detail

This Pull Request changes Arel Nodes In and NotIn to only be marked as preparable = false when they are passed Arrays. Otherwise they do not modify the preparable collector state.

Because Arel is not a public API, I have not created a changelog entry. Happy do so if requested.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.