extendDeep for defaultConfig so we clone edits and don't modify it

Author: alexcjohnson

src/plot_api/plot_api.js

@@ -397,7 +397,7 @@ function opaqueSetBackground(gd, bgColor) {
 }
 
 function setPlotContext(gd, config) {
-    if(!gd._context) gd._context = Lib.extendFlat({}, Plotly.defaultConfig);
+    if(!gd._context) gd._context = Lib.extendDeep({}, Plotly.defaultConfig);
     var context = gd._context;
 
     var i, keys, key;