code: public void headerOut(String key, String value)
    {
      Objects.requireNonNull(value);
      
      if (isOutCommitted()) {
        return;
      }
  
      if (headerOutSpecial(key, value)) {
        return;
      }
      
      setHeaderOutImpl(key, value);
    }
docs: Sets a header, replacing an already-existing header.
  
  @param key the header key to set.
  @param value the header value to set.