combobutton doesn't have isOpen() so zkbind can't read open property

<zk>
	<vlayout apply="org.zkoss.bind.BindComposer" viewModel="@id('vm') @init('org.zkoss.zktest.bind.comp.Combobutton')">
		<combobutton label="combobutton" open="@bind(vm.open)" >
			<popup>
				<vbox>
					<hbox>
						Search
						<textbox />
					</hbox>
					<listbox width="200px">
						<listhead>
							<listheader label="Name" />
							<listheader label="Description" />
						</listhead>
						<listitem>
							<listcell label="John" />
							<listcell label="CEO" />
						</listitem>
						<listitem>
							<listcell label="Joe" />
							<listcell label="Engineer" />
						</listitem>
					</listbox>
				</vbox>
			</popup>
		</combobutton>
		<label id="open" value="@load(vm.open)" />
	</vlayout>
</zk>