fix #886 ColumnHider on OSX disappeared, with latest css changes

We were adding the dgrid-header class to the headerScrollNode, which will hide the ColumnHider icon on OSX browsers that hide the scrollbars by default, because we set overflow:hidden in dgrid-header.

This is regression that appeared in v0.3.14, it was not present in v0.3.13.

fixes https://github.com/SitePen/dgrid/issues/886