Fix "Cannot supply multiple client credentials." error for Okta users

[ArtyomVancyan]

Motivation:

Fixed "Cannot supply multiple client credentials." error for Okta users. This error was found during the investigation in the scope of the #45 discussion. The error occurred because of the Okta-specific implementation of OAuth2 and was fixed based on the discourse conversation.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you updated the documentation related to the changes you have made?
• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?