ikiwiki/ bugs/ special characters in tag names need manual escaping

Having read i18ncharactersinposttitle, I have a page named St John's in a file named St_John__39__s.mdwn. Regular wikilinks like \[[St_John's]] successfully point to that page. However, if I tag a page with [[tag St_John's]], that link is shown as pointing to a non-existant page. Modify the tag to read [[tag St_John__39__s]] works around the problem.

done in 1.49 --Joey