Added a test for encoding an invalid multibyte sequence

The method `Decoda::escape()` using `htmlentities()` or `htmlspecialchars()`
should always replace the invalid multibyte sequences with U+FFFD (UTF-8)
or &#FFFD; (other encodings).
Fixed the encoding an invalid multibyte sequence

`ENT_SUBSTITUTE` should always be used with `htmlentities() / htmlspecialchars()`.