Item12363: use the header row to decide how many columns there are supposed to be in the new row - if someone's hand broken the table, there's no reason to continue the broken ness? (or is there)

git-svn-id: http://svn.foswiki.org/trunk/core@16441 0b4bb1d4-4e5a-0410-9cc4-b2b747904278