shims: Set the client metatable earlier.

Once the signals get propagated, it means "manage" will call code before
the metatable is set. If this happens and it sets some properties, they
will perpetually bypass the `awful.client.object` handler.

tests/examples/shims/client.lua

@@ -140,13 +140,15 @@ function client.gen_fake(args)
 
     client.focus = ret
 
+    setmetatable(ret, {
+        __index     = function(...) return meta.__index(...) end,
+        __newindex = function(...) return meta.__newindex(...) end
+    })
+
     client.emit_signal("manage", ret)
     assert(not args.screen or (args.screen == ret.screen))
 
-    return setmetatable(ret, {
-                        __index     = function(...) return meta.__index(...) end,
-                        __newindex = function(...) return meta.__newindex(...) end
-                    })
+    return ret
 end
 
 function client.get(s)