Conditional attributes are not properly bound

[borismus]

Component markup:

<p>
<label for="labels">Show labels</label>
<input type="checkbox" id="labels" checked?="{{labels}}">
</p>

Implementation with

labels: false,  
labelsChanged: function() {
  console.log('labels is', this.labels);
},

Clicking the checkbox does not cause labelsChanged to fire or labels to update. More complete example: http://jsbin.com/igEwaGO/1/edit

I'm assuming that this should work based on https://github.com/Polymer/TemplateBinding/blob/master/examples/how_to/conditional_attributes.html

/cc @rafaelw

[borismus]

User error. Using = instead of ?= works.