FIX `deepMerge_` was changing the `DEFAULT_OPTIONS` in prototype
FIX `deepMerge_` was changing the `DEFAULT_OPTIONS` in prototype

The `deepMerge_` function was changing the class prototype
(`prototype.DEFAULT_OPTIONS`). Among other things, this was changing the
options used by new instances.

Fixes: #2

rarylson/geochart-geojson,rarylson/geochart-geojson