Disabled fieldsets don't disable input fields on IE (11)

Created by: juristr

Hi,

just discovered that IE (tested v11) doesn't seem to like disabled fieldsets as shown in this demo: http://getbootstrap.com/css/#forms-control-disabled

<fieldset disabled>
...
</fieldset>

While it works on Chrome and FF, IE behaves like this:

Also, it seems as if the problem is only on input fields. The dropdown is disabled as intended.

(Note: I discovered this kind of behavior because some JS unit test failed on PhantomJS which also doesn't seem to interpret disabled fieldsets properly)