Allow to unbind onload/onerror callback handlers

Assigning `null` to handlers removes them, this mimic the behaviour from Google Chrome.

From Chrome's Developer Tools:

```js
> function handler(evt) { console.log('img handler: %s', evt.type); }
< undefined
> var img = document.createElement('img')
< undefined
> img.onload = handler
< handler(evt)
> img.onerror = handler
< handler(evt)
> img.src = 'https://www.google.com/images/errors/logo_sm_2.png'; true
< true
< img handler: load
> img.src = 'https://www.google.com/images/errors/logo_sm_2.png'; true
< true
< img handler: load
> img.src = 'https://example.com/404.png'; true
< true
< img handler: error
> img.src = 'https://example.com/404.png'; true
< true
< img handler: error
> img.onload = null
< null
> img.onerror = null
< null
> img.src = 'https://www.google.com/images/errors/logo_sm_2.png'; true
< true
> img.src = 'https://example.com/404.png'; true
< true
```

src/Image.cc

@@ -268,6 +268,12 @@ NAN_SETTER(Image::SetOnload) {
   if (value->IsFunction()) {
     Image *img = Nan::ObjectWrap::Unwrap<Image>(info.This());
     img->onload = new Nan::Callback(value.As<Function>());
+  } else if (value->IsNull()) {
+    Image *img = Nan::ObjectWrap::Unwrap<Image>(info.This());
+    if (img->onload) {
+      delete img->onload;
+    }
+    img->onload = NULL;
   }
 }
 
@@ -292,6 +298,12 @@ NAN_SETTER(Image::SetOnerror) {
   if (value->IsFunction()) {
     Image *img = Nan::ObjectWrap::Unwrap<Image>(info.This());
     img->onerror = new Nan::Callback(value.As<Function>());
+  } else if (value->IsNull()) {
+    Image *img = Nan::ObjectWrap::Unwrap<Image>(info.This());
+    if (img->onerror) {
+        delete img->onerror;
+    }
+    img->onerror = NULL;
   }
 }
 

test/image.test.js

@@ -150,4 +150,56 @@ describe('Image', function () {
     image.src = new Buffer(0);
     image.src = new Buffer('');
   });
+
+  it('should unbind Image#onload', function() {
+    var img = new Image
+      , onloadCalled = 0;
+
+    img.onload = function() {
+      onloadCalled += 1;
+    };
+
+    img.src = png_checkers;
+    assert.equal(img.src, png_checkers);
+    assert.strictEqual(true, img.complete);
+    assert.strictEqual(2, img.width);
+    assert.strictEqual(2, img.height);
+
+    assert.equal(onloadCalled, 1);
+
+    onloadCalled = 0;
+    img.onload = null;
+    img.src = png_clock;
+    assert.equal(img.src, png_clock);
+    assert.strictEqual(true, img.complete);
+    assert.strictEqual(320, img.width);
+    assert.strictEqual(320, img.height);
+
+    assert.equal(onloadCalled, 0);
+  });
+
+  it('should unbind Image#onerror', function() {
+    var img = new Image
+      , onerrorCalled = 0;
+
+
+    img.onload = function() {
+      assert.fail('called onload');
+    };
+
+    img.onerror = function() {
+      onerrorCalled += 1;
+    };
+
+    img.src = png_clock + 's1';
+    assert.equal(img.src, png_clock + 's1');
+
+    assert.equal(onerrorCalled, 1);
+
+    onerrorCalled = 0;
+    img.onerror = null;
+    img.src = png_clock + 's3';
+    assert.equal(img.src, png_clock + 's3');
+    assert.equal(onerrorCalled, 0);
+  });
 });