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