Split Item::send() into virtual method in Type_handler

bool Item::send(Protocol *protocol, String *buffer)

{

  ...

  switch ((f_type=field_type())) {

  default:

  case MYSQL_TYPE_NULL:

  case MYSQL_TYPE_DECIMAL:

  ...

};

virtual bool Item_send(Item *item, Protocol *p, st_value *buf) const= 0;

virtual bool send(Protocol *protocol, st_value *buffer)

{

  return type_handler()->Item_send(this, protocol, buffer);

}