There have been some changes in the types involved. The object returned by Response/Request.getAttributes().get("org.restlet.http.headers") is a Series<Header>, not a Form, which is a Series<Parameter>. Because of type erasure, there is no runtime problem until the Restlet Engine tries to make use of the Parameter you stored and treat it as a Header.
You'll have to re-write your code to do something like this:
private static final String HEADERS_KEY = "org.restlet.http.headers";
Wow you just saved my life. Everbody and their mother was still posting on the internet that you need to define a Form for the headers. This sort of deprecated information should somehow be reviewed and removed if it's false.