Changing one header cell changes all the header cells on the left

[alinastarkov]

This only happened once when the cell is first changed.

[Marc-Andre-Rivet]

The expected behavior here would be that since these headers are not merged, changing the value of a single header should only change that header!

Further more, if rows is changed to foo, all the header rows will be changed to foo with the current implementation because rows is a string, not an array in column.name. If column.name was [rows, rows, rows] it would change to [rows, foo, rows] when editing the 2nd header row for example.

Changes required

• modifying unmerged header cell affects only that cell, not other columns

• modifying unmerged header cell affects only that cell, not other rows

• merged behavior remains unchanged for columns

• merged behavior is consistent with unmerged behavior across rows

• unit tests for the behavior above on dash-table/units/actions.js

• a sanity integration test for the merged and unmerged behaviors

If in the future merge_duplicate_headers includes merging across rows than the behavior would need to be further adjusted.