Fix polymorphic type setter

Author: Exelord

lib/jsonapi/resource.rb

@@ -324,7 +324,7 @@ def _replace_polymorphic_to_one_link(relationship_type, key_value, key_type, opt
       relationship = self.class._relationships[relationship_type.to_sym]
 
       _model.public_send("#{relationship.foreign_key}=", key_value)
-      _model.public_send("#{relationship.polymorphic_type}=", key_type.to_s.classify)
+      _model.public_send("#{relationship.polymorphic_type}=", _model_class_name(key_type))
 
       @save_needed = true
 
@@ -404,6 +404,12 @@ def _replace_fields(field_data)
       :completed
     end
 
+    def _model_class_name(key_type)
+      type_class_name = key_type.to_s.classify
+      resource = self.class.resource_for(type_class_name)
+      resource ? resource._model_name.to_s : type_class_name
+    end
+
     class << self
       def inherited(subclass)
         subclass.abstract(false)