CSV multi-select field headers do not sit correctly.

Similar to #181967.

When viewing the CSV results for multiple-select fields, the column headers are not separated correctly.

Currently:

+------------+------------+
| Field 1    + Field 2    | 
+------------+------------+------------+------------+------------+------------+
| F1 Value 1 + F1 Value 2 | F1 Value 3 | F2 Value 1 + F2 Value 2 | F2 Value 3 |
+------------+------------+------------+------------+------------+------------+
| Yes        |            | Yes        |            | Yes        |            |
+------------+------------+------------+------------+------------+------------+

Expected result:

+------------+------------+------------+------------+------------+------------+
| Field 1    +            |            | Field 2    |            |            |
+------------+------------+------------+------------+------------+------------+
| F1 Value 1 + F1 Value 2 | F1 Value 3 | F2 Value 1 + F2 Value 2 | F2 Value 3 |
+------------+------------+------------+------------+------------+------------+
| Yes        |            | Yes        |            | Yes        |            |
+------------+------------+------------+------------+------------+------------+

Apply the attached patch from the webform/components directory.

The patch was created by a colleague.