code: @Override
      protected List<String> retrieveValue(final Request req) {
          return req.getParameters().getValues(this.paramName);
      }
docs: Retrieves a parameter (defined in {@link #ParameterRequestMatcher(org.hamcrest.Matcher, java.lang.String)})
  of the given request. The values are percent-encoded.
  @param req request to retrieve the parameter from
  @return the request parameter as a list of values or {@code null} if there is no such a parameter in the request