restyle zmin & zmax doesn't cause contour maps to redraw

[alexcjohnson]

1. open http://localhost:3000/devtools/test_dashboard/#electric_heatmap - changing colorscale works correctly:

Plotly.restyle(gd, {zmin: -50, zmax: 150})

2. Instead, first change to a contour map and change zmin/zmax - we don't see the new range:

Plotly.restyle(gd, {type: 'contour', zauto: true})
Plotly.restyle(gd, {zmin: -50, zmax: 150})

3. but then if you cause a redraw (doubleclick or something) then you do see the new range.