Fetching#nil?

It already does that because nil responds to neither `#to_hash` nor `#to_ary` (it responds to `#to_h` and `#to_a`).  Hence Fetching.from(nil) will return nil.

Of course, that can lead to potential other issues with Fetching -- is `#to_h` an alias for `#to_hash`?  Or are there things that respond to `#to_h` but not `#to_hash`?