use ensure_loaded! with mixin objects

[ccutrer]

This handles the case of a module including itself into the recipient, defined a different file

Completed Tasks

• I have read the [Contributing Guide][contrib].
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[lsegal]

Sorry about the delay. Thanks for the PR!