Fixed js-land instanceof

causes issues with several Canvas modules

lib/context2d.js

@@ -150,7 +150,7 @@ Context2d.prototype.setTransform = function(){
  */
 
 Context2d.prototype.__defineSetter__('fillStyle', function(val){
-  if (val instanceof CanvasGradient) {
+  if ('CanvasGradient' == val.constructor.name) {
     this.lastFillStyle = val;
     this._setFillPattern(val);
   } else if ('string' == typeof val) {
@@ -176,7 +176,7 @@ Context2d.prototype.__defineGetter__('fillStyle', function(){
  */
 
 Context2d.prototype.__defineSetter__('strokeStyle', function(val){
-  if (val instanceof CanvasGradient) {
+  if ('CanvasGradient' == val.constructor.name) {
     this.lastStrokeStyle = val;
     this._setStrokePattern(val);
   } else if ('string' == typeof val) {
@@ -315,7 +315,7 @@ Context2d.prototype.getImageData = function(x, y, width, height){
  */
 
 Context2d.prototype.createImageData = function(width, height){
-  if (width instanceof ImageData) {
+  if ('ImageData' == width.constructor.name) {
     height = width.height;
     width = width.width;
   }