Fixed: Context2d setters with invalid values ignored

lib/context2d.js

@@ -165,6 +165,7 @@ Context2d.prototype.setTransform = function(){
  */
 
 Context2d.prototype.__defineSetter__('fillStyle', function(val){
+  if (!val) return;
   if ('CanvasGradient' == val.constructor.name 
     || 'CanvasPattern' == val.constructor.name) {
     this.lastFillStyle = val;
@@ -192,6 +193,7 @@ Context2d.prototype.__defineGetter__('fillStyle', function(){
  */
 
 Context2d.prototype.__defineSetter__('strokeStyle', function(val){
+  if (!val) return;
   if ('CanvasGradient' == val.constructor.name 
     || 'CanvasPattern' == val.constructor.name) {
     this.lastStrokeStyle = val;
@@ -221,6 +223,7 @@ Context2d.prototype.__defineGetter__('strokeStyle', function(){
  */
 
 Context2d.prototype.__defineSetter__('font', function(val){
+  if (!val) return;
   if ('string' == typeof val) {
     var font;
     if (font = parseFont(val)) {
@@ -252,6 +255,7 @@ Context2d.prototype.__defineGetter__('font', function(){
  */
 
 Context2d.prototype.__defineSetter__('textBaseline', function(val){
+  if (!val) return;
   var n = baselines.indexOf(val);
   if (~n) {
     this.lastBaseline = val;