title: Can't add keys with dots (key paths) to a Mongoose field with type: Schema.Types.Mixed
body: <p>If I try to add some JSON that looks like this:</p>
  
  <pre><code>{ 'character.treasure_chests': 1 }
  </code></pre>
  
  <p>Mongoose barks with an error like this:</p>
  
  <blockquote>
    <p>[Error: key character.treasure_chests must not contain '.']</p>
  </blockquote>
  
  <p>Is this the expected behavior ?</p>