Firebase Emulator doesn't sign idToken, trips web-auth-library up

[CmdrDats]

I'm using the Firebase Authentication Emulator, and kept getting this error locally from my CloudFlare Worker:

[FetchError: Public key "undefined" not found.]

After digging through the code a while and manually using jose to decode the idToken header (where the "undefined" comes from), I discovered that there is no kid on the locally generated idToken - more info here firebase/firebase-tools#2764

I could workaround this by using jose directly when in emulator mode, but it would be ideal if I could provide an 'emulator' flag in the options and the library would simply skip the signing check? A note about the emulator in the readme would also help future users.