def require_that guard, message, return_guard = false
      if not guard then
        raise ExpectationFailedError.new(message)
      end
      if return_guard == true then
        guard
      elsif return_guard != false then
        return_guard
      end
    end
def require_that guard, message, return_guard = false
      if not guard then
        raise ExpectationFailedError.new(message)
      end
      if return_guard == true then
        guard
      elsif return_guard != false then
        return_guard
      end
    end
Require that a guard condition passes

 Simply checks that the guard is truthy, and throws an error otherwise
 @see #require_non_empty