Cell/column insertion in header cell with nested tables

[Jonathan]

Steps to reproduce

1. Create table
2. Create table with header on first line into any of the first table's cells
3. Put the focus in any of the header cells and try to add a column/cell

Expected result

Cell/column added in the current table

Actual result

Cell/column added to the parent table

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

Reproduced on the demo on the website

[Jakub Ś]

DUP of #11192.