title: Mongoose findOne works with "_id" fails with "username"
body: <p>User model:</p>
  <pre class="lang-js prettyprint-override"><code>var User = new Schema({
    username: String,
    password: String
  })
  </code></pre>
  <p>There is a user with the username &quot;six&quot;.</p>
  <p>In mongo shell this works:</p>
  <pre class="lang-js prettyprint-override"><code>db.users.findOne({username:'six'})
  </code></pre>
  <p>With mongoose this works:</p>
  <pre class="lang-js prettyprint-override"><code>User.findOne({_id: req.params.id}, function(err, user) {
    res.json(user)
  })
  </code></pre>
  <p>But this doesn't:</p>
  <pre class="lang-js prettyprint-override"><code>User.findOne({username:'six'}, function(err, user) {
    if(err) console.log(err);
    res.json(user);
  });
  </code></pre>
  <p>There is no error returned but no user either. I am missing something and I can't see what.</p>