use hoverinfo skip instead of none for invisible ggplotly() colorbar trace, closes #1381

[cpsievert]

See #1381

[schloerke]

So the grid lines were taking the hover info first, even though they are underneath the tile?

[cpsievert]

So the grid lines were taking the hover info first, even though they are underneath the tile?

No. In plotly.js, colorbars are per trace, but in ggplot2 they are per plot, so when ggplotly() draws a colorbar, it's connected to an invisible trace that is tied to the scales domain (created by this gdef2trace() function). Since that invisible trace's is on top of the heatmap, it's hover properties are overriding the hover for that (1,1) cell (fyi, you can run plotly_json() on that example to see all the attributes for the invisible trace).