Update JWTSigner.java

Following the standard http://self-issued.info/docs/draft-ietf-oauth-json-web-token.html the header has to contain a "typ" attribute. The current implementation uses "type". I think this is a bug.
There is no easy way to fix this in a 3rd party application because all relevant methods are private. encodedHeader cannot by overridden.

I propose to fix the issue by changing header.put("type", "JWT"); to header.put("typ", "JWT"); in JWTSigner.java.