Wrong formatted CSV when splitting tables

[markov00]

Kibana version:
7.6 and master

Describe the bug:

When creating a Data Table visualization with a split table aggregation, the exported CSV is generated without the first header (the field used in the split-table aggregation), shifting the existing headers by 1 position to the left.

Steps to reproduce:

1. Create a Data Table Visualization
2. Add a split table aggregation
3. click on any Export: Formatted to export the formatted CSV
4. check the missing first header and the shifted headers (as in screenshot)

Expected behavior:
The first header on the exported/formatted CSV should represent the field of the split-table

Screenshots (if relevant):

Errors in browser console (if relevant):
n/a

Any additional context:

The raw export instead works correctly

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)

[tsullivan]

Hi, Data Table visualization isn't integrated with any Reporting Services right now.

Removing reporting-services label.