* Extend image_tag to accept ActiveStorage's Attachments and Variants
* Flip resolve_image_source around
* Add tests for the new use-cases of image_tag
* Remove the higher-level test
* Update image_tag documentation
* Add error states into the test suite
* Re-raise polymorhic_url's NoMethodError as ArgumentError
* delegate_missing_to will raise DelegationError instead of NoMethodError