Merge pull request #294 from derbyjs/fix-contexts-unloadAll

Fix Contexts#unloadAll to account for addition of toJSON method

Author: ericyhwang

Diff for: lib/Model/contexts.js

@@ -34,7 +34,9 @@ Model.prototype.unload = function(id) {
 Model.prototype.unloadAll = function() {
   var contexts = this.root._contexts;
   for (var key in contexts) {
-    contexts[key].unload();
+    if (contexts.hasOwnProperty(key)) {
+      contexts[key].unload();
+    }
   }
 };