* v0.8.1
* Add failing tests for `Headers`
There's currently a bug with `Headers`:
if a header is created with uppercase characters, it can never be found.
The problem is that we only look for lowercase characters in the
`Lookup` instance for `Headers`.
* Convert `Headers` to use `CaseInsensitiveString`
In an effort to be more true to HTTP,
we make the header keys case-insensitive.
This fixes the issue of looking up a header where the casing is different,
Since `CaseInsensitiveString`s compare in a way that ignore casing.
The API for consumers for `Headers` stays the same,
but we get more correct code.
A win for all!
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