[RFR] Fixes arrow in sorted column header when field use sortBy props

[louisbl]

Fixes #2306

[fzaninotto]

Nice! Would you mind adding a test?

[louisbl]

@fzaninotto Where do I add the test?
In the
list integration test

[fzaninotto]

yes, exactly

[louisbl]

@fzaninotto I've added some integration tests for the sort arrow on column header.

The last commit is a bit problematic: in order to really test this bug fix I had to modify the simple demo example to add the sortBy props on the tags ReferenceField.

The problem is that the ra-data-fakerest does not seem to support sorting by a reference field so the rows are actually not sorted.

[fzaninotto]

That will do. Thanks!