Mass import: Switching CSV separator character discards manually added metadata columns

[solth]

Describe the bug
After uploading a CSV file in the mass import form, any additionally added metadata columns added manually using the "+" button are discarded when switching the CSV separator character.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Mass import'
2. Upload an appropriate CSV file
3. Manually add metadata column using the "+" button in the table header
4. Change the "CSV separator character"
5. See error

Expected behavior
Changing the CSV separator character should not discard other new metadata added manually via the "+" button.

Screenshots

Release
3.7 and prior

Additional context
see https://github.com/kitodo/kitodo-production/wiki/Massenimport for details on how to use the mass import feature in Kitodo.Production