def assert_valid_keys(options, valid_options)
      unknown_keys = options.keys - valid_options.keys
      raise(ArgumentError, "Unknown options(s): #{unknown_keys.join(", ")}") unless unknown_keys.empty?
    end
def assert_valid_keys(options, valid_options)
      unknown_keys = options.keys - valid_options.keys
      raise(ArgumentError, "Unknown options(s): #{unknown_keys.join(", ")}") unless unknown_keys.empty?
    end
Tests if +options+ includes only valid keys. Raises an error if
 any key is not included within +valid_options+.
 +valid_options+ is a Hash that must include all accepted keys. values
 aren't taken into account.