Can ENV not be forced to lowercase? Or Case could add a support for converting to AA_BB instead of AA BB

[Zzaniu]

let mut key = key.to_lowercase() Because I see that ENV is forced to be lowercased here, if my struct is renamed to uppercase and underscored, the ENV parsing will fail consistently. I tried using convert_case(config::Case::Upper) but it fails if there is an underscore. Now in order to support env, I have to make it all lowercase and underlined.