Changed 'relaxerror' method to compare Strings

`option.relaxerror` entries were being compared as regular expressions, which doesn't appear to be necessary, and would fail when trying to relax an error like `'& did not start a character reference. (& probably should have been escaped as &amp;.)'` that includes unescaped RegEx characters.

Have altered the `checkRelaxed()` method to compare Strings instead.