Delegate #to_key to #key_factory.

Allows key factories to return to_key values that are usable with rails
when they do not to_s in a pretty fashion.

lib/toy/identity/abstract_key_factory.rb

@@ -9,6 +9,10 @@ def next_key(object)
         raise NotImplementedError, "#{self.class.name}#next_key isn't implemented."
       end
 
+      def to_key(object)
+        [object.id] if object.persisted?
+      end
+
       def eql?(other)
         self.class == other.class && key_type == other.key_type
       end

lib/toy/object.rb

@@ -17,5 +17,9 @@ module Object
     def persisted?
       false
     end
+
+    def to_key
+      key_factory.to_key(self)
+    end
   end
 end