You have to "fool" FM a bit to make this look the way you want.
Make your table without the top ruling in the heading cells.
Then add an extra "top ruling" just above the table, either by
using a tight anchored frame with a line in it, or a specially
designed table with a very "thin" single row with the ruling you want.
You also need to tweak the space above/below settings to make
the frame/table sit just above the "real" table.
If you use a table title above the table you may have to redesign
that or place the table title outside the table, or in the small extra
table above the real table.
--
/Thomas Michanek