Fix: Convert scopes set to list in Credentials.__init__

This change ensures that if a set of scopes is passed to Credentials, it is converted to a list. This prevents issues with JSON serialization (to_json failure) and ensures consistent mutability behavior for the scopes property.

Also updated tests/test__oauth2client.py to handle the type change (set -> list) in equality checks.

Fixes #1145

Author: google-labs-jules[bot]