Add new cloneJSON method for cloning an object

This is useful in tests where `deepMerge` would retain references to old
objects.

app/assets/javascripts/discourse-common/addon/lib/object.js

@@ -3,6 +3,8 @@ function isObject(obj) {
 }
 
 // a fairly simple deep merge based on: https://gist.github.com/ahtcx/0cd94e62691f539160b32ecda18af3d6
+// note: this approach might reference the original object. If you mutate an object once you've deep
+// cloned it, say in a test, it might remain modified. Consider `cloneJSON` instead.
 export function deepMerge(...objects) {
   function deepMergeInner(target, source) {
     Object.keys(source).forEach((key) => {
@@ -53,3 +55,7 @@ export function deepEqual(obj1, obj2) {
     return true;
   }
 }
+
+export function cloneJSON(obj) {
+  return JSON.parse(JSON.stringify(obj));
+}