perf(Scope): optimize $watchCollection when used for watching objects

Since we control the oldValue, we don't need to worry about proto-inhereted properties which means we can use
'for in' and skip hasOwnProperty checks.

http://jsperf.com/for-in-vs-object-keys2

Author: IgorMinar

src/ng/rootScope.js

@@ -562,16 +562,18 @@ function $RootScopeProvider(){
             for (key in newValue) {
               if (newValue.hasOwnProperty(key)) {
                 newLength++;
-                if (oldValue.hasOwnProperty(key)) {
-                  bothNaN = (oldValue[key] !== oldValue[key]) &&
-                      (newValue[key] !== newValue[key]);
-                  if (!bothNaN && (oldValue[key] !== newValue[key])) {
+                newItem = newValue[key];
+                oldItem = oldValue[key];
+
+                if (key in oldValue) {
+                  bothNaN = (oldItem !== oldItem) && (newItem !== newItem);
+                  if (!bothNaN && (oldItem !== newItem)) {
                     changeDetected++;
-                    oldValue[key] = newValue[key];
+                    oldValue[key] = newItem;
                   }
                 } else {
                   oldLength++;
-                  oldValue[key] = newValue[key];
+                  oldValue[key] = newItem;
                   changeDetected++;
                 }
               }
@@ -580,7 +582,7 @@ function $RootScopeProvider(){
               // we used to have more keys, need to find them and destroy them.
               changeDetected++;
               for(key in oldValue) {
-                if (oldValue.hasOwnProperty(key) && !newValue.hasOwnProperty(key)) {
+                if (!newValue.hasOwnProperty(key)) {
                   oldLength--;
                   delete oldValue[key];
                 }