Change favicon_link_tag helper mimetype from image/vnd.microsoft.icon to image/x-icon.

Although the official IANA-registered MIME type for ICO files is image/vnd.microsoft.icon,
registered in 2003, it was submitted to IANA by a third party and is not recognised by Microsoft products.
The MIME type image/x-icon should be used since is the one recognised by the major browsers on the market.