Replace automatic inheritance of all bindingContext properties with n…

…ew $parentContext variable

Benefit #1: Can reach custom properties on *all* ancestor contexts, and are not limited to those that haven't been overwritten
(e.g., if we had $index, would now be able to reach all ancestor $index values, not just the closest one)
Benefit #2: Allows discrimination between properties at the current level and properties at the parent level, which
is good because not all custom properties may make sense to inherit.

spec/bindingAttributeBehaviors.js

@@ -322,7 +322,7 @@ describe('Binding attribute syntax', {
         ko.applyBindings(null, testNode);
         value_of(ko.contextFor(testNode.childNodes[1].childNodes[0])).should_be(innerContext);
         value_of(ko.contextFor(testNode.childNodes[1].childNodes[1]).$parent).should_be(innerContext.$data);
-        value_of(ko.contextFor(testNode.childNodes[1].childNodes[1]).custom).should_be(true);
+        value_of(ko.contextFor(testNode.childNodes[1].childNodes[1]).$parentContext.custom).should_be(true);
     },
 
     'Should not reinvoke init for notifications triggered during first evaluation': function () {

src/binding/bindingAttributeSyntax.js

@@ -2,17 +2,17 @@
     ko.bindingHandlers = {};
 
     ko.bindingContext = function(dataItem, parentBindingContext) {
+        this['$data'] = dataItem;
         if (parentBindingContext) {
-            // copy all properties from parent binding context
-            ko.utils.extend(this, parentBindingContext);
+            this['$parentContext'] = parentBindingContext;
             this['$parent'] = parentBindingContext['$data'];
-            this['$parents'] = (this['$parents'] || []).slice(0);
+            this['$parents'] = (parentBindingContext['$parents'] || []).slice(0);
             this['$parents'].unshift(this['$parent']);
+            this['$root'] = parentBindingContext['$root'];
         } else {
             this['$parents'] = [];
             this['$root'] = dataItem;        	
         }
-        this['$data'] = dataItem;
     }
     ko.bindingContext.prototype['createChildContext'] = function (dataItem) {
         return new ko.bindingContext(dataItem, this);