Remove superfluous lowercasing of feature names

[Tak]

Fixes #716

I'm a little apprehensive that the lowercasing was there for a reason, but all the tests are passing 🤷

[dandavison]

Thanks very much @Tak!

I'm a little apprehensive that the lowercasing was there for a reason

I've thought about it but I'm not sure why I did that. I think I may have had in mind code I'd written to do something like support "Red" and "red" as color names (which we do support), and then added that canonicalization inappropriately here. I don't see a reason why case-sensitivity of feature names will confuse people, and indeed it seems appropriate.