title: Moshi equivalent of Gson's "serializeNulls"
body: <p>I recently replaced Moshi for Gson in a backend that's expected to serialize responses with nullable values as <code>{ "value": null }</code>, instead of <code>{}</code>. </p>
  
  <p>Neither Moshi nor Gson do this by default, but Gson has an option to do it directly in the builder: </p>
  
  <p><code>Gson gson = new GsonBuilder().serializeNulls().create()</code>. </p>
  
  <p>Does Moshi have support for something similar?</p>