stop adding iat in payload (optional claim)

[jbidzik]

fix #111
As mentioned in RFC7519, iat is an optional claim.
If iat is added in payload, token that does not contains iat will never pass signature verification.

[odino]

hey @jbidzik will this create backward compatibility problems?

[odino]

Also, sorry for the delay for having a look at this....busy :P

[jbidzik]

@odino i dont think this will create BC break because we stop adding iat on both token generation and validation.
However, if someone want to add iat claim, its not possible anymore.