Test and fix for issue with belongs_to in CastedModel

lib/couchrest/model/casted_model.rb

@@ -53,6 +53,13 @@ module CouchRest::Model
     end
     alias :to_key :id
     alias :to_param :id
+
+    # Fixes belongs_to assocations. 
+    # See it "should allow to reference association by id in casted model" in 
+    # assocations_spec.rb
+    def model_proxy 
+      nil
+    end
     
     # Sets the attributes from a hash
     def update_attributes_without_saving(hash)