When merging header cell with body cell, table tools options for merge should be disabled

[Jakub Ś]

Steps to reproduce

1. Create 3x3 table with header in 1st row
2. Fill table cells with sample text (numbers for example)
3. Click inside middle header cell and merge it down

Expected result

Header cell should not be merged with body cell because the end result doesn't make sense IMHO. The structure of the table doesn't chnage that much (header is higher and one body cell gets removed - I think it is far from expected) There should be no result or menu should be disabled.

Actual result

Cells get merged. Header cell is higher (has rowspan=2), in first body row there are two cells and last row has 3 cells.
It really looks like one cell was removed and table is broken (although end result is correct).

Other details (browser, OS, CKEditor version, installed plugins)

Please see attached video.