fix: convert input to lowercase during arg parse

[damaredayo]

Fixes #842

[MaxOhn]

Not quite as simple with rust unfortunately 😩
str::to_lowercase allocates a new String which is dropped right after taking its reference and rust doesn't like that.

I'd recommend using the more efficient CowUtils::cow_to_ascii_lowercase to avoid the forced reallocation but it will have the same issue regarding returned references.

To fix that, I recommend not modifying the parse_key method and instead lowercase the key_opt value that results from the parse_key call.

[damaredayo]

Sorry, been writing too much go recently I think - silly error on my part. Just getting back into rust after hyper-focusing go in my career, appreciate you being as friendly as always Max <3

[MaxOhn]

Don't forget to also adjust the parse_any function which handles arguments of the form AR9.3 instead of AR=9.3 😉

Would also appreciate if you add test cases at the bottom regarding lowercasing.