fix: polymorphic on expects a symbol (#433)

Fix polymorphic type mismatch in polymorphic_belongs_to by ensuring symbol conversion

Author: jasonkarns

lib/graphiti/sideload/polymorphic_belongs_to.rb

@@ -41,7 +41,7 @@ def ignore?(group_name)
     end
 
     def on(name, &blk)
-      group = Group.new(name)
+      group = Group.new(name.to_sym)
       @groups << group
       group
     end