Bug fix for assignment of customProperties in the option template

[AnomalousBit]

Description

When providing a JSON encoded data-custom-properties attribute on the original <option> elements, attempting to access the dataset.customProperties later, inside of a Choices event would return an improperly encoded "[object Object]" string instead of the expected JSON object.

This PR fixes the option template attribute for data-custom-properties by using JSON.stringify() before attempting to covert customProperties into a string.

Several issues open around custom properties may be fixed by this PR.

Screenshots (if appropriate)

Types of changes

• Chore (tooling change or documentation change)
• Refactor (non-breaking change which maintains existing functionality)
• 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 not work as expected)

Checklist

• My code follows the code style of this project.
• I have added new tests for the bug I fixed/the new feature I added.
• I have modified existing tests for the bug I fixed/the new feature I added.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[tagliala]

Should fix #1036

[mtriff]

Thanks for this! Can you add a unit test for this that would have failed previously but now passes with your change?