title: Mongoose does not update values
body: <p>I am trying to update a field's value in my MongoDB with mongoose. However, it does not update it correctly.</p>
  
  <p>Here is what I have so far:</p>
  
  <pre><code>Test.findById(987456, function(err, doc) {
      if (doc) {
          var map = doc.data;
          map['2019-07-07'] = {
              TAS: "111",
              TWS: "222",
              TSWD: "333"
          }
          doc.set('data', map)
          doc.save((err, doc1) =&gt; {
               if(err) console.log(err);
               else{
                console.log(doc1)
               }
         });
      }
  })
  </code></pre>
  
  <p>This is my Schema</p>
  
  <pre><code>var test = new Schema({
      data: { type: Schema.Types.Mixed, default: {} },
  })
  var Test = mongoose.model('test', test);
  </code></pre>
  
  <p>The data prior to updating inside the <code>data</code> field is <code>{ '2019-06-06': { TWS: '4', TAS: '27', TSWD: '33' }</code></p>
  
  <p>The code does not throw any errors and <code>console.log(doc1)</code> inside the save callback prints the correct value <code>{ '2019-06-06': { TWS: '4', TAS: '27', TSWD: '33' },'2019-07-07': { TAS: '111', TWS: '222', TSWD: '333' }}</code></p>
  
  <p>However, when I check MongoDB Atlas, the value is not changed at all.</p>