fix: convert all arguments to lowercase

[lze3]

If you're checking the lower-case value is included within an array that may not be lower-case, that's pointless and may result in cases where the condition is never true. For example, you want to match an upper-case string in a string[], but because this checks lower-case, it will never be true.

[Gawdl3y]

All items in oneOf should be specified as lowercase. This is to avoid having to call toLowerCase on args every time like in this PR. Ideally, there should probably be validation on construction to verify that oneOf doesn't contain any non-lowercase entries (or just make them all lowercase at that time).