fixedHeaderOptions not working when selectableRows='none'

Thanks for the feature, it is really useful.

## Expected Behavior
When `selectableRows == 'none'` the first column (whatever it is) should stay in view when scrolling horizontally with setting
```javascript
fixedHeaderOptions: {
xAxis: true
}
```
## Current Behavior
When the first column isn't the row selection checkboxes column- the first column doesn't stay in view when you scroll horizontally.

## Steps to Reproduce (for bugs)
1. Take the `fixedHeaderOptions` demo.
2. Scroll horizontally- notice the first column of checkboxes stays in view even when you scroll.
3. Add `selectableRows: 'none'` to `options`.
4. Now scroll horizontally again. The first column (which is now the "Name" column) doesn't stay in view.

## Your Environment
| Tech | Version |
|--------------|---------|
| Material-UI | 3.9.2 |
| MUI-datatables | 2.13.0 |
| React | 16.8.4 |
| browser | Chrome 78 |