title: dynogels update trashing other keypairs
body: <p>Anyone with dynogels experience might be able to help me on this one.</p>
  
  <p>Simplified example of my dynamodb table with a nested structure</p>
  
  <pre><code>{
    key: xxxxx,
    maintenance: {
      date1: xxxxxxxx,
      date2: xxxxxxxx
    }
  }
  </code></pre>
  
  <p>If I update the table and send the below as the update params</p>
  
  <pre><code>key: 1,
  maintenance: {
    date2: 1970-01-18T09:45:55.452Z
  }
  </code></pre>
  
  <p>then date1: gets trashed from my item in the table</p>
  
  <p>Is there some config option in the update call that Im making that I'm missing somewhere to NOT delete values that I dont want to touch/update?</p>
  
  <p>Thanks</p>