Previous code unconditionally treated all cells in merge groups as
rows span, thus in some conditions it merged incorrectly when there
were cells from same row.

The fix is inspired by SwTable::PrepareMerge().

Also, fixed improper adjustment of too narrow cells in
WW8TabDesc::CalcDefaults(), which also happens in files where
column spans are represented by spanned 1-twip-wide cells.

Unit test included.

Change-Id: I6c3803635f006dd396805d6847daa4d80c97cad4
Reviewed-on: https://gerrit.libreoffice.org/34820Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>