Search and csv export functionalities with expanded rows.

Hi,

I have a doubt about the expandable rows. I have a table with expandable rows and it's totally funcional.
The unique problem that i found was the csv exports and the search not process the expanded rows.
To solve the search case i thougth that i can use no visible columns to put there the key columns of the expanded rows. But to solve the csv export i don't know, without process the data out of the table component, what i can do.
Someone had the same problem? I read the doc and i played with the examples of "onDownload" but i coudn't access to the expanded rows data.

## Expected Behavior
I hope that the expanded rows datas could be processed.

## Current Behavior
Search and csv export functionalities only process the non expanded rows.

## Steps to Reproduce (for bugs)

## Your Environment

| Tech | Version |
|--------------|---------|
| Material-UI | 4.2.0 |
| MUI-datatables | 2.6.3 |
| React | 2.5.0 |
| browser | Chrome Versión 71.0.3578.98 |
| etc | |