Allow `kid` to be optional in JWT header

The current use of `github.com/MicahParks/keyfunc` requires a `kid` in the JWT header. This isn't strictly required and will return an error today if the `kid` field is missing.

The error returned:
```
the JWT has an invalid kid: could not find kid in JWT header
```

https://github.com/MicahParks/keyfunc/blob/f76c64fa36d9b7e4a96547313dcb651d54aa006b/keyfunc.go#L17-L24
