Sortable property ignored if no field is set for EuiTable

[chrisronline]

It appears that if you do not set a field property on a EUI table column, the column is not sortable at all.

This seems like a bug to me because a valid use case for not setting a field property is for computed columns. In this use case, the sortable prop supports a function callback so you can easily recompute the same value in the callback.