Fix error when using byContentVal decorator

[cuberoot]

I was getting an error about writing to a read-only property when I used @byContentVal

Description

I found that the decorators that worked for me marked the property as writable. I fixed byContentVal to do the same.

Related Issue

Motivation and Context

How Has This Been Tested?

I ran the included unit tests

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x ] I have signed the Adobe Open Source CLA.
• [ x] My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• [ x] All new and existing tests passed.