Make has_many through singular associations build CPK records

[gmcgibbon]

Motivation / Background

This Pull Request has been created because composite primary key records cannot be build when using has_many through singular (has_one) associations.

Detail

This Pull Request changes build_record on ActiveRecord::Associations::ThroughAssociation to support composite primary key associations.

Additional information

I noticed some odd behaviour for build_record that I'll be submitting fixes for in separate patches.

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.

[gmcgibbon]

Hm, I think this should actually include has_ones through has_ones as well. I'll add another test and update the description.

Never mind, that doesn't work because of

rails/activerecord/lib/active_record/associations/through_association.rb

Line 97 in 2d6f523

raise HasOneThroughCantAssociateThroughHasOneOrManyReflection.new(owner, reflection)

[eileencodes]

Can you fix the conflicts?

[gmcgibbon]

Fixed!