Use _read_attribute when autosaving has_one associations

[gmcgibbon]

Motivation / Background

This Pull Request has been created because composite primary key models do not autosave properly when their association primary key is set to :id. In models with composite primary keys, the #id accessor will return an array, where we actually want the id column value.

Detail

This Pull Request changes has_one autosaving to use _read_attribute instead of public_send so that it won't use the #id accessor.

Additional information

Are we concerned with this breaking behaviour of existing apps?

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.

[eileencodes]

I'm not sure if the test failures are related to this or from another PR that went in today. Can you take a look at the failures?

[gmcgibbon]

Oh yeah, I remember this, some DBs don't like nil PKs. Will fix, thanks