Added a test for escaping a string with an invalid multibyte sequences

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 escaping a string with an invalid multibyte sequences

`ENT_SUBSTITUTE` should always be used with `htmlentities() / htmlspecialchars()`.
Merge pull request #83 from alquerci/fix/security/escape

Fixed escaping a string with an invalid multibyte sequences