Add more unit tests.

[DFurnes]

We should try to get more test coverage on Gateway! Some things that come to mind:

• [x] Testing client authenticated requests
• [ ] Add tests for Northstar client's public API:
  • [x] user index
  • [x] get user (by term)
  • [ ] update user
• [ ] Testing user authenticated requests
• [ ] Testing error handling for invalidated refresh tokens
• [ ] Testing if invalid scopes or credential errors are handled properly
• [ ] Testing that JSON responses are serialized into response classes properly
• [ ] Unit testing the response classes with fake data (independent of HTTP responses)
• [ ] Add tests for the ForwardsTransactionIds trait

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[DFurnes]

(13) As a developer, I'd like to make more unit tests for Gateway