title: remove ReactiveDict values
body: <p>This is how I define some ReactiveDict variable:</p>
  
  <pre><code>this.templateDictionary = new ReactiveDict();
  
  this.templateDictionary.set( 'type', type );
  this.templateDictionary.set( 'size', size );
  this.templateDictionary.set( 'bgcolor', bgcolor );
  this.templateDictionary.set( 'color', color );
  </code></pre>
  
  <p>Now I want to remove everything:</p>
  
  <pre><code>this.templateDictionary.set( 'type', undefined );
  this.templateDictionary.set( 'size', undefined );
  this.templateDictionary.set( 'bgcolor', undefined );
  this.templateDictionary.set( 'color', undefined );
  </code></pre>
  
  <p>But if I do a console.log, I see the variable get the string <code>undefined</code>. So it isn't really removed.</p>
  
  <p>And by the way: Is it possible to set multiple values by one line?</p>