TypeError: Cannot create property 'iat' on string

Repro against jsonwebtoken 8.3.0, repros against master:
```
$ node --use_strict
> const JWT=require('jsonwebtoken')
undefined
> JWT.sign('asdf','aa')
TypeError: Cannot create property 'iat' on string 'asdf'
    at Object.module.exports [as sign] (/Users/test/backend/node_modules/.registry.npmjs.org/jsonwebtoken/8.3.0/node_modules/jsonwebtoken/sign.js:144:17)
```

Fixed by #577 