code: public Href with(final Object key, final Object value) {
          final SortedMap<String, List<String>> map = new TreeMap<>(this.params);
          if (!map.containsKey(key.toString())) {
              map.put(key.toString(), new LinkedList<>());
          }
          map.get(key.toString()).add(value.toString());
          return new Href(this.uri, map, this.fragment);
      }
docs: Add this extra param.
  @param key Key of the param
  @param value The value
  @return New HREF