Fix form_for id generation for new CPK models

[gmcgibbon]

Motivation / Background

This Pull Request has been created because dom ID generation for CPK models is subtly broken.

Detail

This Pull Request changes Action View by adding tests for form_for when using composite primary key models and fixes dom_id output for new CPK models.

Additional information

Every record key casting is either going to be nil or an array, so the code should be safe.

Also, I removed the original CompositePrimaryKeyRecord because it didn't follow the existing pattern of using example-based models in tests. As a result, I updated the test using it to use the new fake CPK model.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated 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.