What does this implement/fix? Explain your changes.

• Add test for Decimal default parameter
• Drop outdated work around to fix decimal default parameters

Does this close any currently open issues?

Fixes #1769

Checklist

Check all those that are applicable and complete.

• Make sure to include one or more tests for your change
• If an enhancement PR, please create docs and at best an example
• Ensure you have signed the .NET Foundation CLA
• Add yourself to AUTHORS
• Updated the CHANGELOG