def quote(string)
      assert_type string, :String, :string
      if string.type != :string
        quoted_string(string.value)
      else
        string
      end
    end
def quote(string)
      assert_type string, :String, :string
      if string.type != :string
        quoted_string(string.value)
      else
        string
      end
    end
Add quotes to a string if the string isn't quoted,
 or returns the same string if it is.

 @see #unquote
 @example
   quote("foo") => "foo"
   quote(foo) => "foo"
 @overload quote($string)
   @param $string [Sass::Script::Value::String]
 @return [Sass::Script::Value::String]
 @raise [ArgumentError] if `$string` isn't a string