draggable-pieces property works differently then in documents

[cartereric]

Hello Justin,

Thank you for this element. I've used it in a project that can be seen here http://polyglot-spa-root-config.s3-website.us-east-2.amazonaws.com/

I found an issue in the docs that say the property draggable-pieces can be set to "true" or "false" but this is not the behavior described https://lit-element.polymer-project.org/guide/properties#conversion. The presence of the property no matter the value makes the pieces draggable. To prevent dragging the pieces the property has to be set to NULL or undefined.