message: Fix broken `class="rustdoc"` handling
  Before, it was generating code like this:
  ```html
  <div class="rustdoc mod" container-rustdoc="" id="rustdoc_body_wrapper" tabindex="-1">
  ```
  Now it generates code like this:
  ```html
  <div class="rustdoc mod container-rustdoc" id="rustdoc_body_wrapper" tabindex="-1">
  ```
diff: @@ -27,7 +27,15 @@ pub(crate) fn rewrite_lol(
  // ```
  let body_handler = |rustdoc_body_class: &mut Element| {
  // Add the `rustdoc` classes to the html body
  - rustdoc_body_class.set_attribute("container-rustdoc", "")?;
  + let mut tmp;
  + let klass = if let Some(classes) = rustdoc_body_class.get_attribute("class") {
  + tmp = classes;
  + tmp.push_str(" container-rustdoc");
  + &tmp
  + } else {
  + "container-rustdoc"
  + };
  + rustdoc_body_class.set_attribute("class", klass)?;
  rustdoc_body_class.set_attribute("id", "rustdoc_body_wrapper")?;
  rustdoc_body_class.set_attribute("tabindex", "-1")?;
  // Change the `body` to a `div`
  