- #2664
- use new property to correctly determine `isTargetEnum` in `GetCurrentValues()`
- avoid `ArgumentNullException` in all cases where raw values are `enum` but target is not
- stop skipping tests blocked by #2664, exposing a couple more #1487 issues
- use new property instead of private `GetElementType()` methods where possible
- cleans up some duplicate code
- also remove redundant use of `IsCollectionType` and `ElementMetadata`
nits:
- move properties above methods in `ModelMetadata`
- avoid accidentally-incorrect "Remove Unnecessary Usings"
