awful.client.floating.toggle: Fix for dialogs

awful.client.floating.get() does some sensible defaults. E.g. dialogs are
floating by default. Since floating.toggle only checked the property, these
defaults made it break.

So floating.toggle() should use floating.get to decide if a given client is
floating. It should also use "false" instead of "nil" when the client is made
non-floating or else the default will apply again.

Thanks to vsp for making me figure this out. :)

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/client.lua.in

@@ -627,8 +627,8 @@ end
 function floating.toggle(c)
     local c = c or capi.client.focus
     -- If it has been set to floating
-    if property.get(c, "floating") then
-        floating.set(c, nil)
+    if floating.get(c) then
+        floating.set(c, false)
     else
         floating.set(c, true)
     end