export canvas.CanvasPixelArray instead of canvas.PixelArray which is undefined

lib/canvas.js

@@ -13,7 +13,7 @@ var canvas = require('./bindings')
   , Canvas = canvas.Canvas
   , Image = canvas.Image
   , cairoVersion = canvas.cairoVersion
-  , PixelArray = canvas.PixelArray
+  , PixelArray = canvas.CanvasPixelArray
   , Context2d = require('./context2d')
   , PNGStream = require('./pngstream')
   , JPEGStream = require('./jpegstream')

test/canvas.test.js

@@ -76,6 +76,10 @@ module.exports = {
     }
   },
 
+  'test .PixelArray': function(){
+    assert.equal(typeof Canvas.PixelArray, 'function');
+  },
+
   'test color serialization': function(){
     var canvas = new Canvas(200, 200)
       , ctx = canvas.getContext('2d');