code: public List<UserDto> selectByEmail(DbSession dbSession, String emailCaseInsensitive) {
      return mapper(dbSession).selectByEmail(emailCaseInsensitive.toLowerCase(ENGLISH));
    }
docs: Search for an active user with the given emailCaseInsensitive exits in database
  
  Select is case insensitive. Result for searching 'mail@emailCaseInsensitive.com' or 'Mail@Email.com' is the same