Cannot change the alternative text for free-standing images

Follow this steps to reproduce:

• Edit the Sandbox home page
• Scroll to the end of the page where the XWiki logo image is found
• Edit the image and change only the alternative text. Leave everything unchanged.
• Save or switch to source.

The source syntax is still:

image:XWikiLogo.png

The problem is that this is a free-standing image and since CKEditor doesn't remove the 'wikimodel-freestanding' CSS class from the image element when the alternative text is changed the XWiki rendering ignores the new value of the alt attribute.