Fix uppercase lowercase isses

[YounessBird]

fix #340

[YounessBird]

I have chosen the first approach, which is to use lowercase keys which will make the crate case insensitive, this will allow the user to use uppercase or lowercase keys but internally the keys can be easily matched and overridden since they are all lowercase. As of the caveat I mentioned here I managed to fix it by matching the fields and the keys for lowercase in the deserialization phase.

[pictographer]

Looks good.

One suggestion. Mention that keys are case insensitive in the docs.

[YounessBird]

Thanks. I will add your suggestion to the chore list 😄

[YounessBird]

Done and ready for a review.

[matthiasbeyer]

So first of all, thanks for working on this. This is important to get right.

I have some remarks, nothing too critical.

Still, I am not sure whether this is the right way to go. Converting all things to lowercase seems ... I don't know to be honest.

I am willing to merge this (modulo some things that have to be cleared), if we do not find another way... that said, when (if) we re-do the crate (#321) we should get it right from the get-go and support case sensitivity as well as insensitivity (let the user decide). But that's of course for another day, I guess.

[matthiasbeyer]

This was laying around way too long. I'm gonna merge this now, sorry for me taking so long!

[utkarshgupta137]

There is still a bug (as of 0.13.3).
Consider this:

let conf: HashMap<String, String> = Config::builder()
    .add_source(File::with_name(name))
    .add_source(Environment::with_prefix(prefix))
    .build()?
    .try_deserialize()?;

File:

KEY = "value1"

Env:

PREFIX_KEY=value2

Then:
Current behavior: conf has 2 keys: KEY & key with values value1 & value2
Expected behavior: conf has 1 key: KEY with value value2