When requesting a configuration property in a context that infers `&str`, the method fails

[mehcode]

let redis_client = redis::Client::open(settings.get("redis_url").unwrap()).unwrap();

invalid type: string "...", expected a borrowed string

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This is because of type conversions. We must return a String because there may have been a type conversion. I can see this fixed in two ways:

• Fix this by allowing to get out a &str if the underlying value is stored as a String. I don't like this because it'd only work sometimes.
• Fix this by storing the type conversion in the cache when the value is requested. This makes a kind of sense because I believe we can optimize for users requesting the same type of value over-and-over-again. I was already going to add an initial "guess" that losslessly converts your incoming config value on the assumption that if you give us a "1" through an environment variable you probably meant 1.

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As a side note I'm starting a medium-large project in Rust (finally) and am using this crate. Hopefully will find a ton of usage issues and annoyances (like this).