Unify the way character encoding is selected

[iain-logan]

There is a discrepancy between the encoding and decoding of JWT's when it comes to character encoding.

Encoding is always done in UTF-8, decoding however defaults to UTF-8 but can be set to other supported character encodings.

Some type that represents the character encoding to use will be created, an instance of which will be optionally provided when decoding and encoding a JWT. If no such argument is provided, we default to UTF-8.