There were two issues here:

1) c3b0f135 added a mechanism to work
around broken documents, but that didn't deal with nested tables.
Additionally, the check compared the max row width with the width of the
last cell, not the last row.

2) d276d3f3 cleaned up implicit
horizontal merge detection, but didn't deal with valid documents, where
each row definition is available twice.

Change-Id: I1a32ec2fcb0d1a8fca7a7bd26501d3daf17880c9