Interceptor bind client

[erikfried]

Description

the client was not bound to getSigningKey in interceptor.js, making this undefined in the JwksClient instance.
This was causing a TypeErrorwhen the interceptor would not return a key.

     TypeError: Cannot read property 'logger' of undefined
      at getSigningKey (src/JwksClient.js:71:10)
      at JwksClient.<anonymous> (src/wrappers/interceptor.js:26:12)
      at async Context.<anonymous> (tests/interceptor.tests.js:39:19)

References

Testing

Unit tests added to reproduce the error, and verify the fix.

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master

[davidpatrick]

Thanks @erikfried