Add unit test for jwt.Sign() variadic parameter options

[KendallWeihe]

Originally thought of while reviewing #89

We should add a test to jwt_test.go something like...

func TestSign_Options(t *testing.T) { ... }

Which will test the variadic parameter options. Type() will be easy to unit test but Purpose() not as straight forward because (I think?) we'll need to generate a DID with multiple keys and instruct the signing to occur on the non-default key (a lot of assumptions in that statement; just briefly looking at it, it's complicated enough to stub in a ticket to investigate)

[KendallWeihe]

Probably we need unit tests for all our variadic parameter options all over the place, not just JWT. Feel free to use this ticket as a chance to create more tickets.