Fix @setModel logic so that @sync is called even on root-only keypaths.

@@ -48,10 +48,11 @@ class Rivets.Binding
 
       model = current
 
-    if @key and @model and @model isnt model
-      @unbind true
+    if @model
+      if @model isnt model
+        @unbind true if @key
         @model = model
-      @bind true
+        @bind true if @key
         @sync()
     else
       @model = model