Inline style doesn't support background-image

[fernandomm]

Steps to reproduce

1. Create link with the following content in full featured editor ​http://ckeditor.com/demo#full:

<a href="http://google.com" style="background-image: url('http://example.com')">Example</a>

2. Try to edit the link in "Link" toolbar button
3. When you click Ok, you will get the following message:

Value specified for the inline style must consist of one or more tuples with the format of "name : value", separated by semi-colons.

Expected result

The inline style is valid and should be accepted.

Actual result

You can't edit the link unless the background-image is removed.

[Jakub Ś]

DUP of #12197