canvas.getContext with alpha:undefined should behave like alpha:true

LayoutTests/fast/canvas/alpha.js

 description("Series of tests for canvas alpha");
 
 if (window.testRunner)
     testRunner.dumpAsTextWithPixelResults();
 
 var canvas1 = document.getElementById("canvas1");
 var canvas2 = document.getElementById("canvas2");
 var canvas3 = document.getElementById("canvas3");
 var canvas4 = document.getElementById("canvas4");
+var canvas5 = document.getElementById("canvas5");
+var canvas6 = document.getElementById("canvas6");
 
 var ctx1 = canvas1.getContext("2d");
 var ctx2 = canvas2.getContext("2d", {} );
 var ctx3 = canvas3.getContext("2d", { alpha: false } );
 var ctx4 = canvas4.getContext("2d", { alpha: true } );
+var ctx5 = canvas5.getContext("2d", { alpha: undefined } );
+var ctx6 = canvas6.getContext("2d", { alpha: null } );
 
 shouldBe("ctx1.getContextAttributes().alpha", "true");
 var imgData1 = ctx1.getImageData(0, 0, 1, 1);
 shouldBe("imgData1.data[0]", "0");
 shouldBe("imgData1.data[1]", "0");
 shouldBe("imgData1.data[2]", "0");
 shouldBe("imgData1.data[3]", "0");
 
 shouldBe("ctx2.getContextAttributes().alpha", "true");
 var imgData2 = ctx2.getImageData(0, 0, 1, 1);
@@ skipping 16 matching lines @@
 // getContextAttributes().
 var attrs = ctx4.getContextAttributes();
 shouldBe("attrs.alpha", "true");
 attrs.alpha = false;
 var imgData4 = ctx4.getImageData(0, 0, 1, 1);
 shouldBe("ctx4.getContextAttributes().alpha", "true");
 shouldBe("imgData4.data[0]", "0");
 shouldBe("imgData4.data[1]", "0");
 shouldBe("imgData4.data[2]", "0");
 shouldBe("imgData4.data[3]", "0");
+
+shouldBe("ctx5.getContextAttributes().alpha", "true");
+shouldBe("ctx6.getContextAttributes().alpha", "true");