Removed invalidIssuedAt

[realmike33]

major problem for production severs

[kylef]

I'd like to go down the route with a leeway instead of completely removing this validation.

#54 (comment)

If the leeway doesn't work for you, you can disable validation and manually call the validate*() functions on the returned claim set that you would like to use, for example:

let claimset = decode(..., validate: false)
claimset.validateIssuer("x")
claimset.validateExpiary()