title: d3 selection .attr, .property, etc setter methods' behaviour when value is undefined
body: <p>What is the expected behaviour when I call <code>d3.select("input[type=checkbox]").property("checked", undefined)</code>?</p>
  
  <p>Should it be treated as falsy and thus the <code>checked</code> property set to <code>false</code>/removed, or should it be treated as no value and thus like calling <code>.property("checked")</code>, the getter?</p>