Ensure _.isPlainObject returns false for objects with a custom [[Prototype]]. [closes #2085]

lodash.js

@@ -10607,7 +10607,7 @@
       if (proto === null) {
         return true;
       }
-      var Ctor = proto.constructor;
+      var Ctor = hasOwnProperty.call(proto, 'constructor') && proto.constructor;
       return (typeof Ctor == 'function' &&
         Ctor instanceof Ctor && funcToString.call(Ctor) == objectCtorString);
     }

test/test.js

@@ -10856,6 +10856,18 @@
       }
     });
 
+    QUnit.test('should return `false` for objects with a custom `[[Prototype]]`', function(assert) {
+      assert.expect(1);
+
+      if (create) {
+        var object = create({ 'a': 1 });
+        assert.strictEqual(_.isPlainObject(object), false);
+      }
+      else {
+        skipAssert(assert);
+      }
+    });
+
     QUnit.test('should return `false` for DOM elements', function(assert) {
       assert.expect(1);