Fix EZP-21930: searchreplace plugin generates an infinite loop while changing the case in IE9/10

JIRA: https://jira.ez.no/browse/EZP-21930

# Description

This is actually a TinyMCE bug, I reported it in http://www.tinymce.com/develop/bugtracker_view.php?id=6528 and this PR is the same as the one I proposed upstream https://github.com/tinymce/tinymce/pull/281

# Tests

Manual test in IE 8/9/10 (In IE8 I did not notice any regression and in IE 9/10 the bug is of course fixed).