[IE] Radio selected parameter not added correctly

[Marek Lewandowski]

since: 3.6.6.1 (didn't check earlier) until master

checked only on IE10, but this issue might be also present in other IE versions

1. open any sample with CKEditor (i.e. samples/replacebyclass.html)
2. use "Radio Button" button from toolbar
3. set Name to "foo"
4. check "Selected" option checkbox
5. confirm by clicking "OK"
6. check source

Expected result:
Following source code: <p><input name="foo" type="radio" checked="checked" /></p>

Current result:
Code without checked attribute: <p><input name="foo" type="radio" /></p>

additional info:

1. it seems that removing attribute works correctly, because if you put expected source code and you'll go to edit dialog - option is properly preselected, and attribute is being removed if you will uncheck "Selected" option checkbox and update it
2. some internal properties might be stored correctly, because proper image representation (in editor content) is picked (depending if you chose it to be selected or not)

[Jakub Ś]

DUP of #8175.