"iat" needs to be a number

[levino]

"iat" needs to be a number

[levino]

I decided to release the fix for this as a "non-breaking change". It is "breaking" in a sense because code will now error that sets "iat" or "exp" to strings (usually stringified numbers). However I think that this is a very rare usecase for this library since using expiration in unit tests would require fuzzing with the clock which is always hard. Even if someone hits this error, maybe it is a good thing to require them changing these values from strings to numbers after all and these changes should not take too long and are pretty obvious and easy to find. I hope I did not create too much of a fall out. If I did, please complain here.