code: public void add(HttpHeader header, String value) {
          if (value == null)
              throw new IllegalArgumentException("null value");
  
          HttpField field = new HttpField(header, value);
          add(field);
      }
docs: Add to or set a field. If the field is allowed to have multiple values,
  add will add multiple headers of the same name.
  
  @param header the header
  @param value  the value of the field.