"Link To" setting for images is no longer sticky

[designsimply]

Steps to reproduce:

1. Clear browser cache
2. Log in to WP blog and open any post for editing
3. Open the browser console and check the cookie setting for urlbutton (see snippet below)
4. Click the "Add Media" button and click on an image in the Media Library
5. Look for the "Link To" setting in the Attachment Display Settings at bottom right
6. Set "Link To" to None
7. Check cookie setting
8. Click the "Insert into post" button
9. Check the cookie setting
10. Click the "Add Media" button
11. Check the cookie setting
12. Does the setting in the UI match the cookie setting

• Expected: "Link To" setting to stick
• Actual: "Link To" setting doesn't stick

Screencast: ​https://cloudup.com/cMdu-FMTtkO (Length: 2:42)

Seen at ​http://localhost/trunk/wp-admin/post.php?post=1&action=edit using Firefox 31.0 on Mac OS X 10.9.4 and WP r29112

Cookie console snippet (props azaozz):

document.cookie.split('; ').forEach( function(a){ console.log( decodeURIComponent(a).match(/urlbutton=.*/g) ) } );

[ocean90]

Duplicate of #23847.