Add UserProfile.toMap and unit test

[MilesAdamson]

• [x] All new/changed/fixed functionality is covered by tests (or N/A)
• [x] I have added documentation for all new/changed functionality (or N/A)

📋 Changes

• Added UserProfile.toMap() method
• Added call to the above function in Credentials.toMap() to add the result in there

📎 References

https://github.com/auth0/auth0-flutter/issues/379

🎯 Testing

Unit test added which converts to/from map and checks the result