Bugfix for PRE-Tag and Sub-Tags

Version 5.7.2 introduced a bug while trying to fix ”< (less-than sign)
inside a PRE element, and NOT start of a valid tag, was incorrectly
removed” - closing tags inside the pre-tag are now invalid and look
like e.g. “&lt;/span>”

This should be pushed upstream.
Merge pull request #131 from gamma/master

Bugfix for PRE-Tag and Sub-Tags