TableDiff: Unicode characters are silently dropped from diff #136

Problem description

The TableDiff first converts some of the string's characters into named entities using htmlentities(). The remaining characters are tried to be converted using iconv() (might behave differently depending on underlying system) from UTF-8 into ISO-8859-1.

In 0.1.16 this failed with Notice: iconv(): Detected an illegal character in input string if characters remained that have no equivalent in ISO-8859-1 and neither modifiers //TRANSLIT or //IGNORE have been used.

In 0.1.17 there was a change (see #134 and its corresponding PR) that added the iconv modifier //IGNORE. While the conversion now succeeds, it silently drops any characters that have no representation in ISO-8859-1.

In our application there is an editorial workflow that needs to rely on the diff output being complete, so silently dropping characters isn't an viable option.

Steps to reproduce

$old = '<table><tr><td>React with :-) and "joy".</td></tr></table>';
$new = '<table><tr><td>React with 🙂 and "joy".</td></tr></table>';

$htmlDiff = new HtmlDiff($old, $new);
echo $htmlDiff->build();

In 0.1.16 it fails (or might fail), in 0.1.17 the Unicode character 🙂 is silently dropped.

Possible solution

PR #137