fix: preserve property case for custom elements

[socsieng]

Fixes #5184

Heuristics used to determine whether a custom element's property or attribute should be set doesn't support properties that include uppercase characters. This PR preserves property casing when dealing with custom elements.

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Tests

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dummdidumm]

Thank you, but this isn't the right fix. The correct fix (which is implemented since a while now - sorry for letting this go unanswered for so long) is to set the property through an attribute instead if the property doesn't exist on the element, and the custom element is responsible for transforming attributeChanged callbacks correctly; therefore closing.