awful.rules: fix client tagging with multiple screens

Commit a757dda replaced the awful.client.movetotag call with c:tags()
in the tag property handler. Which introduced a regression when more
than one screen is used. The awful.tag.withcurrent function was being
called even when there was a valid tag property. Now the clients
screen is set to the tag screen to ensure the client is moved to the
correct tag and screen.

Signed-off-by: Adrian C. (anrxc) <anrxc@sysphere.org>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/rules.lua.in

@@ -110,6 +110,7 @@ function apply(c)
             aclient.floating.set(c, value)
         elseif property == "tag" then
             c:tags({ value })
+            c.screen = value.screen
         elseif property == "switchtotag" and value and props.tag then
             atag.viewonly(props.tag)
         elseif property == "height" or property == "width" or