title: Empty collections not present in JSON request
body: <p>I have a Sinatra app and I'm updating some stuff in a mongoDB database with some PUT-requests.</p>
  
  <p>In javascript, the actual values I'm posting are:</p>
  
  <pre><code>entity: {
      config: someString,
      parameters: parameterArray, // EMPTY [ ]
      dt: dtArray, // EMPTY [ ]
  }
  </code></pre>
  
  <p>I'm doing this via jQuery ajax.</p>
  
  <p>On the server side I get the following:</p>
  
  <pre class="lang-ruby prettyprint-override"><code>params[:entity] #=&gt; config = someString
  </code></pre>
  
  <p>But the empty arrays are missing. Why is this? I need them! The same route will handle both empty arrays and no parameter at all in a different way...</p>