Change Expandable Icon in TableSelectCell to Better Conform to Material

By the way, thank you for all your hard on this great library!

And now for a nitpick:

The icon to activate an expandable row is a right-facing arrow icon that animates clockwise 90 degrees. According to the material specs for collapsibles (https://material.io/design/components/lists.html#implementation, admittedly this applies to lists and not tables, but still sets a good precedent), the arrow should be downward facing for collapsed rows and then animate 180 degrees to the upward position for expanded rows.

## Expected Behavior
Expandable icons should begin in the downward position for collapsed rows and then change to the upward position for expanded rows.

## Current Behavior
Expandable icons begin in the rightward facing position for collapsed rows and then change to the downward position for expanded rows.

Minimal example:
```
expanded
}}
/>
```

It looks like everything needed to do this is in https://github.com/gregnb/mui-datatables/blob/master/src/components/TableSelectCell.js. If you think this would be a good first issue and you accept contributions, I would be happy to submit a PR.

Thank you for reading.