docs: public void addHeader (@Nonnull final String sName, @Nullable final String sValue)
    {
      ValueEnforcer.notNull (sName, "HeaderName");
  
      final String sNameLower = sName.toLowerCase (Locale.US);
  
      m_aRWLock.writeLocked ( () -> {
        ICommonsList <String> aHeaderValueList = m_aHeaderNameToValueListMap.get (sNameLower);
        if (aHeaderValueList == null)
        {
          aHeaderValueList = new CommonsArrayList <> ();
          m_aHeaderNameToValueListMap.put (sNameLower, aHeaderValueList);
          m_aHeaderNameList.add (sNameLower);
        }
        aHeaderValueList.add (sValue);
      });
    }