Improvements to requestWithAutomaticAccessTokenRenewal

[vgritsenko]

This fixes error handling, and makes a couple of improvements to implementation of the requestWithAutomaticAccessTokenRenewal function:

• Pass all error responses as-is, and look only for the tokenExpired;
• Treat onTokenRenewal parameter as truly optional - perform the token renewal even if onTokenRenewal is nil;
• Do not keep a strong reference to self in the completion block for renewAccessToken call.

The main issue I had was that requestWithAutomaticAccessTokenRenewal would swallow all errors and convert them to the empty tokenExpired errors, preventing any possibility of implementing custom error handling logic on the client side:

completion(.failure(.tokenExpired(error: nil)))