title: $unset not working on array of objects?
body: <p>I have a collection with documents like below:</p>
  
  <pre><code>{
     title: "whatever",
     answers: [
        {id: 10, question: ObjectId("54380350a52147000aad4e9b")},
        {id: 13, question: ObjectId("54380350a52147000aad4e9c")},
        {id: 33}
     ]
  }
  </code></pre>
  
  <p>I'm have attempted to unset the <code>question</code> attribute using the commands below:</p>
  
  <pre><code>db.participants.update({}, {$unset: {"answers.question": ""}}, {upsert: false, multi:true} )
  </code></pre>
  
  <p>and</p>
  
  <pre><code>db.participants.update({}, {$unset: {"answers.question": 1}}, {upsert: false, multi:true} )
  </code></pre>
  
  <p>Both of these spit out: <code>WriteResult({ "nMatched" : 628795, "nUpserted" : 0, "nModified" : 0 })</code> when completed, but no documents are updated (as the message suggests).  Any idea why my update isn't working?</p>