Table header are set wrong if they are changed from "Both" to "First Columns only"

[mattalexx]

Steps to reproduce:

1. Create a new table with default options, except choose Headers: Both.
2. Fill in a few rows with fake data.
3. Right click table and choose Table Properties.
4. Under Headers, choose "First Column Only"

I expect the first column of the first row to remain a TH, but it is changed to a TD.

[Jakub Ś]

DUP of #9189.