docs: public boolean contains(HttpString headerName) {
          final HeaderValues headerValues = getEntry(headerName);
          if (headerValues == null) {
              return false;
          }
          final Object v = headerValues.value;
          if (v instanceof String) {
              return true;
          }
          final String[] list = (String[]) v;
          for (int i = 0; i < list.length; i++) {
              if (list[i] != null) {
                  return true;
              }
          }
          return false;
      }