Fix cartesian hover toggle

The hover toggle button couldn't be enabled on cartesian plots because
the later setup completely overwrote the initial allowed groups.  So
just make it add the cartesian buttons instead of replacing the groups
completely.

Author: Sizurka

src/components/modebar/manage.js

@@ -183,7 +183,7 @@ function getButtonGroups(gd) {
     // regardless of what other types are on the plot, since they'll all
     // just treat any truthy hovermode as 'closest'
     if(hasCartesian) {
-        hoverGroup = ['toggleSpikelines', 'hoverClosestCartesian', 'hoverCompareCartesian'];
+        hoverGroup.push('toggleSpikelines', 'hoverClosestCartesian', 'hoverCompareCartesian');
     }
     if(hasNoHover(fullData) || hasUnifiedHoverLabel) {
         hoverGroup = [];