Doc suggestion for tests

[littletime]

Hi,

I've been struggling today with the implementation of my tests with rspec, which would not pass even using the FirebaseIdToken.test! and the pretty cool code snippet explaining how to generate a token in test mode.

In order for the FirebaseIdToken::Signature.verify method to return a payload, it has to be valid, which implies 4 things to be in order in the test payload that's encrypted for token generation:

• iat timestamp value before the test running time
• exp timestamp value after the test running time
• aud string to contain the project id
• iss string to contain a url that depends on the project id

This error is invisible when testing with custom payload that's not a Google-generated token from actual firebase project.

It would be nice to add in the Test section of the documentation something that reminds of those requirements for a valid test payload.