The current implementation of _to_partial_path composes the part of two bits, `collection' and `element'.
ActiveModel::Name also contains these fields, and they are derived the same way _to_partial_path does it.
However, _to_partial_path doesn't use the information in model_name, and solely relies on its own computations instead.
This works for all standard cases, but not necessarily for models that provide a non-standard model_name.

This commit fixes that and has _to_partial_path use model_name if the class responds to it.