JWT Serializer sets default media type header

The JWT IETF draft specifies that there can be a `typ` [header](https://tools.ietf.org/html/draft-ietf-oauth-json-web-token-25#section-5.1) to declare the media type of the JWT. The header is optional but is useful to indicate that the object is a JWT.

Most other JWT libraries set this header to "JWT", this explicitness is generally useful.