Handle stray closing pre tag in ParagraphWrapper.

The source of /ko/오프라_윈프리 seems to have a stray </pre> tag that
ParagraphWrapper couldn't handle. It caused an unnecessary state
transition.

Adds a test for stray closing pre tags.

Change-Id: I1cc00e91ba826d0df0eafefd85cc931855e2b8a6