[minor]Don't update, when image attribute 'src' has changed

Hello, imsky, I have a suggestion: 

When I modified the holder image src attribute, and then the window resize event is triggered, the picture should not be reduced to the svg.

Setp to reproduce:
1. html:
```html
<img src="" id="preview-img" data-src="holder.js/100x200" >
```
2.When I finished uploading a picture, and then I modified the holder image attribute src:
```javascript
...
$('#preview-img').attr('src', 'http://xxxx.jpg');
...
```

3.trigger window resize event.

the image attribue ```src``` will become `data:image/svg+xml;base64,...`

Also, I complement the end of the line ';'.

Thanks!