[crud] Fix columns headers in RTL #834
Description
Column headers cannot be properly specified with rtl languages (i.e. Persian).
Default header labels are generated from the path. https://github.com/vaadin/vaadin-crud/blob/a1a6cc149b6612e2bac5f76e01e6a119f893349b/src/vaadin-crud-grid.html#L137
In case of using RTL language encoding issues do occur.