Prevent calling the callback more than once

[snocorp]

Changes

During the login in Auth0RestClient._request:

• If the callback is passed, do not call resolve or reject as calling reject will invoke the callback again
• if there is an error, return after handling the error instead of invoking the callback again

References

Fixes #758

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• This change adds unit test coverage
• This change adds integration test coverage

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[adamjmcgrath]

Thanks @snocorp!