exportExcel: unnecessary Rowspans break rendering of table

[rdmueller]

Describe the bug

When each cell of a row is merged with the same cell on the next row, a table row like this will be produced:

.2+| ten .2+| eleven

As this seems to be correct, it will break the rendering of the table in Asciidoctor with an error

dropping cell because it exceeds specified number of columns

To Reproduce

Here is an AsciiDoc snippet which reproduces the Asciidoctor bug:

|===
| one | two
| three | four
.2+| five | six
| seven
| eight | nine
.2+| ten .2+| eleven
| next | next
| next | next
|===

An excel file which reproduces the behaviour has to be created.

Expected behavior

The whole table is rendered. I guess it would be enough to remove the rowspans