do not coerce layout.colorscale if not specified

Author: antoinerg

src/plots/plots.js

@@ -1362,9 +1362,11 @@ plots.supplyLayoutGlobalDefaults = function(layoutIn, layoutOut, formatObj) {
     coerce('hidesources');
 
     coerce('colorway');
-    coerce('colorscale.sequential');
-    coerce('colorscale.sequentialminus');
-    coerce('colorscale.diverging');
+    if(layoutIn.colorscale) {
+        if(layoutIn.colorscale.sequential) coerce('colorscale.sequential');
+        if(layoutIn.colorscale.sequentialminus) coerce('colorscale.sequentialminus');
+        if(layoutIn.colorscale.diverging) coerce('colorscale.diverging');
+    }
 
     coerce('datarevision');