RefreshToken() should take an optional refresh token from the caller

[RedChops]

In the case of RetrieveSessionAsync(), if the user's session is expired, we will not have a CurrentSession instance set yet. In this case, the RefreshToken from the expired session should be provided to RefreshToken(). Additionally, RefreshToken() should persist the new session retrieved from the API.

• Note: In the current release, the application will crash in RefreshToken because nothing sets CurrentSession before it's called on initialization; this felt like a cleaner route to provide that dependency rather than setting CurrentSession immediately before calling the function only for it to be changed immediately after. Additionally, there was nothing to persist the newly retrieved sessions so if the application restarted after a refresh, it would be in an unrecoverable state.

[acupofjose]

Great stuff, clean implementation! Appreciate the PR, very helpful!