Fixed display name change for IE8

After saving the display name, the oldDisplayName field's value was
wrongly set with text(), which would append the text inside the input
element which is considered as an invalid operation in IE8.

This fix for #5054 correctly puts the old value into the field with a val() call.
Merge pull request #5281 from owncloud/settings-ie8-changedisplaynamefix

Fixed display name change for IE8