title: ractive warning "Two-way binding does not work with expressions"
body: <p>This is the <a href="http://jsfiddle.net/rn6qwn92/1/" rel="nofollow">demo</a>,when I click the checkbox "chooseAll",There is a warning in console,that say</p>
  
  <p><code>Two-way binding does not work with expressions (`items.length===hasChooedItems().length` on &lt;input&gt;</code>,
   I change the ractive's version form "latest" to "0.6.1",when i click the "chooseAll",there is a wrong in console,that say<code>Cannot read property 'split' of undefined</code>. why?</p>