(fix) refreshTokenWithCompletion does not complete if the variables this.provider.tokenEndpoint or this.tokenResult do not exist

[erodriguezh]

PR Checklist

• [ ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [X] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [X] All existing tests are passing
• [ ] Tests for the changes are included

What is the current behavior?

The Promise returned by the function refreshTokenWithCompletion does not complete if the variables this.provider.tokenEndpoint or this.tokenResult do not exist.

More details in the issue #191

What is the new behavior?

The Promise returned by the function refreshTokenWithCompletion will complete with an error if the variables this.provider.tokenEndpoint or this.tokenResult do not exist.

Fixes/Closes #191

[alexziskind1]

Thanks!