allow to override TemplateProcessor#ensureUtf8Encoded

[nicoder]

the method is protected,
but since it is called with self instead of static
it does not allow for subclasses to override it

(we have a special case of mostly ISO-8859-1 strings sometimes containing MS Word apostrophes,
and these appear as question marks in the document when utf8_encode is used,
but are displayed fine when we use mb_convert_encoding($string, 'UTF-8', 'windows-1252'))

An alternative would be to make the ensureUtf8Encoded non static,
since it only appears to be used in a non static method.

[troosan]

indeed, it would make sense to not make that method static. Being a java developer, it feels wrong to override a static method :-S