title: Save JSON object to MongoDB inside String field
body: <p>I'm working with Mongodb(mongoose) and node.js (express.js).
  The model of DB is:</p>
  
  <pre><code>var messageSchema   = new Schema({
      _channel: { type: Schema.ObjectId, ref: 'Channel', required: true },
      _user : { type: Schema.ObjectId, ref: 'User', required: true },
      datetime: { type: Date, required: true },
      messageType: { type: String, required: true },
      publish: { type: Boolean, default: false },
      content: {
          title: String,
          text: String,
      }
  });
  </code></pre>
  
  <p>I want save JSON object inside text field (string).
  The JSON object is this:</p>
  
  <pre><code>{ event: 'push',
    repository: { id: 53012902, name: 'RestAPI' },
    ref: 'refs/heads/master',
    commits: 
     [ { id: 'a10202e5b5157ae5ccd2d77d7d578046693ae404',
         url: 'https://github.com/1izpena/RestAPI/commit/a10202e5b5157ae5ccd2d77d7d578046693ae404',
         author: '1izpena' } ],
    sender: { login: '1izpena', html_url: 'https://github.com/1izpena' } }
  </code></pre>
  
  <p>I convert this in String, but the result is:</p>
  
  <pre><code>{"event":"push","repository":{"id":53012902,"name":"RestAPI"},"ref":"refs/heads/master","commits":[{"id":"a10202e5b5157ae5ccd2d77d7d578046693ae404","url":"https://github.com/1izpena/RestAPI/commit/a10202e5b5157ae5ccd2d77d7d578046693ae404","author":"1izpena"}],"sender":{"login":"1izpena","html_url":"https://github.com/1izpena"}}
  </code></pre>
  
  <p>And this result is not a String, I need to keep the same format to parse it later as json object. 
  Any idea?
  Many thanks</p>