Which version of the SDK are you using?

1.10.0

A quick summary and/or background

Discovered this issue when debugging refresh token issues with @RettBehrens .
If scope is passed as a str (or another type) rather than a list in the token set to the helper method store_xero_oauth2_token, then refresh_oauth2_token will return None rather than the refreshed token set.

Steps to reproduce

1. Pass scopes as a space delimited string within the token set passed to an store_xero_oauth2_token
2. Call refresh_oauth2_token
3. Verify the return value of refresh_oauth2_token is None

What you expected would happen

1. Pass scopes as a space delimited string within the token set passed to an store_xero_oauth2_token
2. Call refresh_oauth2_token
3. can_refresh_access_token raises an error describing why the token cannot be refreshed: in this case, the error should state that scope must be a list of str

What actually happens

1. Pass scopes as a space delimited string within the token set passed to an store_xero_oauth2_token
2. Call refresh_oauth2_token
3. Since the return value of refresh_oauth2_token is not needed since store_xero_oauth2_token is called internally within refresh_oauth2_token, the issue is invisible and a silent failure occurs
4. The issue becomes known when the access token expires since it can never be refreshed due to failing type validation