Don't unbind onload/onerror callbacks after invoking them

From "[WHATWG HTML Living Standard]
(https://html.spec.whatwg.org/multipage/indices.html#event-load)", the definition of 'onload':
> Fired at the Window when the document has finished loading; fired at an
element containing a resource (e.g. img, embed) when its resource has finished
loading

To adhere to that specification both handlers should be called as many times as Image loads.

From Google 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
```

src/Image.cc

@@ -366,8 +366,6 @@ Image::loaded() {
 
   if (onload != NULL) {
     onload->Call(0, NULL);
-    delete onload;
-    onload = NULL;
   }
 }
 
@@ -381,8 +379,6 @@ Image::error(Local<Value> err) {
   if (onerror != NULL) {
     Local<Value> argv[1] = { err };
     onerror->Call(1, argv);
-    delete onerror;
-    onerror = NULL;
   }
 }
 

test/image.test.js

@@ -7,6 +7,7 @@ var Canvas = require('../')
   , Image = Canvas.Image
   , assert = require('assert');
 
+var png_checkers = __dirname + '/fixtures/checkers.png';
 var png = __dirname + '/fixtures/clock.png';
 
 module.exports = {
@@ -36,6 +37,36 @@ module.exports = {
     assert.equal(1, n);
   },
   
+  'test Image#onload multiple times': function() {
+    var img = new Image
+      , n = 0;
+
+    img.onload = function() {
+      ++n;
+    };
+
+    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);
+
+    img.src = png;
+    assert.equal(img.src, png);
+    assert.strictEqual(true, img.complete);
+    assert.strictEqual(320, img.width);
+    assert.strictEqual(320, img.height);
+
+    assert.equal(n, 2);
+
+    n = 0;
+    img.onload = function() {
+      ++n;
+    };
+    img.src = png;
+    assert.equal(n, 1);
+  },
+
   'test Image#onerror': function(){
     var img = new Image
       , error
@@ -66,6 +97,36 @@ module.exports = {
     assert.equal(1, n);
   },
   
+  'test Image#onerror multiple calls': function() {
+    var img = new Image
+      , n = 0;
+
+
+    img.onload = function() {
+      assert.fail('called onload');
+    };
+
+    img.onerror = function() {
+      ++n;
+    };
+
+    img.src = png + 's1';
+    assert.equal(img.src, png + 's1');
+
+    img.src = png + 's2';
+    assert.equal(img.src, png + 's2');
+
+    assert.equal(n, 2);
+
+    n = 0;
+    img.onerror = function() {
+      ++n;
+    };
+    img.src = png + 's3';
+    assert.equal(img.src, png + 's3');
+    assert.equal(n, 1);
+  },
+
   'test Image#{width,height}': function(){
     var img = new Image
       , n = 0;