Header field 'typ' is never set if createheader called.

```
What steps will reproduce the problem?
1. Instantiate similar to: JsonToken jsonToken = new JsonToken(signer);

2. Fill the object out;

3. Call jsonToken.serializeAndSign();

What is the expected output? What do you see instead?

Should get something like: {"alg":"RS256","typ":"JWT"}

But instead get: {"alg":"RS256"}

What version of the product are you using? On what operating system?

Using trunk (1.0 is similar).

Please provide any additional information below.

Google specifies that for obtaining access tokens on service accounts the "typ"
field must exist. See
https://developers.google.com/accounts/docs/OAuth2ServiceAccount#formingheader
for details.

```

Original issue reported on code.google.com by `tdh...@gmail.com` on 16 May 2013 at 9:42