code: public Map<String, String> additionals() {
          final ImmutableMap.Builder<String, String> builder = ImmutableMap.builder();
          for (Entry<String, String> e : params.entrySet()) {
              if (!DEFINED_PARAM_KEYS.contains(e.getKey())) {
                  builder.put(e.getKey(), e.getValue());
              }
          }
          return builder.build();
      }
docs: Returns additional (or, user-defined) parameters.