Support to add additional JWT headers

[callmepjs]

Creating pull request to add support for adding additional JWT headers to the JWT token.

New headers can be specified in the header property of the payload. For example, to specify a key id (kid) used to sign the JWT token:

   var jwt = require('json-web-token');
   var payload = { iss: 'my_issurer', iat: 1400062400223, header: {"kid": "myKeyId"} };
   jwt.encode(key, payload, 'RS256');

Decoded Header:

  {
     "typ": "JWT",
     "alg": "RS256",
     "kid": "myKeyId"
  }

Decoded Payload:

  {
     "iss": "my_issurer",
     "iat": 1400062400223
  }

[joaquimserafim]

Hi @callmepjs can you add a test for this case please, Thanks

[callmepjs]

Hi @joaquimserafim , added unit test cases for verifying encoding and decoding.

[callmepjs]

Thanks. Could you please make a new release with this patch ?