title: Moshi is not mapping null values in Map
body: <p>Let's say that we have a class that contains only one property:</p>
  
  <pre><code>data class MyClass(val map: Map&lt;String, String?&gt;)
  </code></pre>
  
  <p>As you can see, <code>map</code> property may contain <code>null</code> values, e.g.:</p>
  
  <pre><code>{
      "key1": "value1",
      "key2": null
  }
  </code></pre>
  
  <p>Now if I use this instance of adapter to create JSON:</p>
  
  <pre><code>moshi.adapter(MyClass::class.java)
  </code></pre>
  
  <p>It will skip entries with <code>null</code> values (for the example above it will return a <code>String</code> like this: <code>{ "key1": "value1" }</code>). The same thing happens if I add <code>.serializeNulls()</code> to the adapter instance.</p>
  
  <p><strong>Question</strong></p>
  
  <p>How can I force a Moshi adapter to take <code>null</code> into consideration?</p>