lit: undefined/null attribute gets rendered as 'undefined'

When rendering attributes, switching from a value to undefined causes ‘undefined’ to be set literally in the DOM.

This seems unexpected, how else do we let the renderer know that an attribute should no longer be set? See: http://jsbin.com/qigigavaqu/edit?html,output for a reproduction.

If you inspect the dom, you will first see <div foo="bar"></div>. After clicking the button, you wil see <div foo="undefined"></div>

The same behavior is observed when rendering a null attribute