Check for sameColumns in ColumnMetrics breaks Selectable Cell example

[malonecj]

In commit https://github.com/prometheusresearch/react-grid/commit/ba519b8283845467779f666ebe390cbfb348ea38 a check was added to prevent state from being reset if nextProps columns do not differ from previous ones.

This breaks the example listed here http://prometheusresearch.github.io/react-grid/examples/selectable-cells.html once you use the keyboard navigation to scroll past the current viewport height.

I guess this check should remain here as columnMetrics should be unaware of scroll height. Potentially, set the scrollTop in GridScrollMixin when this occurs so we can re-render grid?