code: @Override
      public void setExtensions(List<ExtensionConfig> extensions) {
          this.extensions.clear();
          if (extensions != null) {
              this.extensions.addAll(extensions);
          }
      }
docs: Set the list of extensions that are approved for use with this websocket.
  <p>
  Notes:
  <ul>
  <li>Per the spec you cannot add extensions that have not been seen in the {@link UpgradeRequest}, just remove entries you don't want to use</li>
  <li>If this is unused, or a null is passed, then the list negotiation will follow default behavior and use the complete list of extensions that are
  available in this WebSocket server implementation.</li>
  </ul>
  
  @param extensions the list of extensions to use.