DropdownInputWidget: Only allow setting values actually in the dropdown
DropdownInputWidget: Only allow setting values actually in the dropdown

This was already done correctly in the PHP widget.

Change-Id: If5860cf40826564a573b4230b41e18ff2e09c88b

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui