title: Distance returns null for Mongodb 2.6
body: <p>I have just upgraded mongodb from 2.4 to 2.6.
  Now when I run the query</p>
  
  <pre><code>$hospitals=$qb-&gt;select('distance','title','date','photos','description')
                  -&gt;field('coordinates')
                  -&gt;near((float)$lat, (float)$lng)
                  -&gt;limit(9)
                  -&gt;getQuery()
                  -&gt;execute()
                  -&gt;toArray();
  </code></pre>
  
  <p>distance is always null. <br>
  is this a bug or something ?
  <br>PS:$lat &amp; $lng are OK!</p>