Maps that are not Enumerable

[bartlomiej-korpus]

Hello!

When using Recase.Enumerable.convert_keys, it tries to recursively convert keys in maps. The problem is that not every map has Enumerable protocol implementation, so it raises if it finds for example DateTime struct.

Guess we could check if Enumerable is implemented?

[sobolevn]

Yes, good idea! PRs are welcome.