Fixed #16721 -- Made sure that blocktrans correctly handles percents (%), even...

Fixed #16721 -- Made sure that blocktrans correctly handles percents (%), even in the plural block. Thanks for the initial patch, Claude Paroz.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16730 bcc190cf-cafb-0310-a4f2-bffc1f526a37