title: Get object keys for filtered values
body: <p>The case is simple - I've got a following object:</p>
  
  <pre><code>Object {1: false, 2: true, 3: false, 4: false, 5: false, 6: false, 7: false, 8: true, 12: false, 13: false, 14: false, 15: false, 16: false, 17: false, 18: false, 19: false} 
  </code></pre>
  
  <p>and I need to get an array of ids that had true value, using underscore. In above case that would be:</p>
  
  <pre><code>[2, 8]
  </code></pre>
  
  <p>I tried few things but I'm a bit stuck. Does anyone have any idea?</p>