code: public RequestParameters copy(String prefix) {
      String keyPrefix = prefix+":";
  
      RequestParameters copy = new RequestParameters();
      for (Map.Entry<String, List<String>> entry: entrySet()) {
        copy.put(keyPrefix+entry.getKey(), entry.getValue());
      }
  
      return copy;
    }
docs: Creates a copy of the parameters, prepending a namespace prefix
  to each parameter name.
  @param prefix	the prefix to prepend
  @return	the copy of the parameters