Safari 'sortable helper' is invisible issue

This issue is quite similar to issue #630 with sortable-helper "behind" popup. But it is specific to Safari. There are few cases when Safari calculates `z-index`-es in strange way and you need to apply css-transformations to force sortable clone appearing above popup.

There is quite simple PR #605 which can help with this issue -- it allows you to apply custom css-transformations to helper and I believe that it can help in my case.

Is there any chances for this PR to be reviewed and merged?

Does anybody know any another solutions for using of `react-sortable-hoc` along with popups and Safari?