Doing "Merge cells" should only preserve the top-left cell value

[tepi]

Steps to reproduce:

1. Create a sheet with different values a A1 and A2
2. Select A1 and A2, right-click and select Merge cells
3. Right-click the merged region and select Unmerge cells

-> The original value of A2 is magically there again. That value should have been deleted at merge time. Additionally if the value in A2 is long enough, it will overflow from under the merged region to the right.