backport angular.copy fix from 1.3.x branch

backported commit:
angular@b59b04f

which is a fix to:
angular#4996

Author: Grahame Bowland

src/Angular.js

@@ -783,7 +783,8 @@ function copy(source, destination, stackSource, stackDest) {
       } else if (isRegExp(source)) {
         destination = new RegExp(source.source);
       } else if (isObject(source)) {
-        destination = copy(source, {}, stackSource, stackDest);
+        var emptyObject = Object.create(Object.getPrototypeOf(source));
+        destination = copy(source, emptyObject, stackSource, stackDest);
       }
     }
   } else {
@@ -818,12 +819,14 @@ function copy(source, destination, stackSource, stackDest) {
         delete destination[key];
       });
       for ( var key in source) {
-        result = copy(source[key], null, stackSource, stackDest);
-        if (isObject(source[key])) {
-          stackSource.push(source[key]);
-          stackDest.push(result);
+        if(source.hasOwnProperty(key)) {
+          result = copy(source[key], null, stackSource, stackDest);
+          if (isObject(source[key])) {
+            stackSource.push(source[key]);
+            stackDest.push(result);
+          }
+          destination[key] = result;
         }
-        destination[key] = result;
       }
       setHashKey(destination,h);
     }

test/AngularSpec.js

@@ -24,6 +24,16 @@ describe('angular', function() {
       expect(copy([], arr)).toBe(arr);
     });
 
+    it("should preserve prototype chaining", function() {
+      var GrandParentProto = {};
+      var ParentProto = Object.create(GrandParentProto);
+      var obj = Object.create(ParentProto);
+      expect(ParentProto.isPrototypeOf(copy(obj))).toBe(true);
+      expect(GrandParentProto.isPrototypeOf(copy(obj))).toBe(true);
+      var Foo = function() {};
+      expect(copy(new Foo()) instanceof Foo).toBe(true);
+    });
+
     it("should copy Date", function() {
       var date = new Date(123);
       expect(copy(date) instanceof Date).toBeTruthy();