Headers are being treated case sensitive

[TimDiam0nd]

As per RFC 4485 header names/keys MUST be case insensitive.
This is generally treated fine in other libs such as requests using a CaseInsensitiveDict object.
Its also (implicitly) done within this library when using a Message object.

[jhnstrk]

👍
There seems to be a typo in the title - it should be "case sensitive" (the headers are currently treated as case sensitive, when they shouldn't be).

[TimDiam0nd]

Good spot haha