prevent double decoration, closes #173

lib/draper/base.rb

@@ -26,7 +26,7 @@ module Draper
     def initialize(input, options = {})
       input.inspect # forces evaluation of a lazy query from AR
       self.class.model_class = input.class if model_class.nil?
-      @model = input
+      @model = input.kind_of?(Draper::Base) ? input.model : input
       self.options = options
     end
 

spec/draper/base_spec.rb

@@ -44,6 +44,11 @@ describe Draper::Base do
       ProductDecorator.new(source).model_class.should == Product
     end
 
+    it "returns decorator if it's decorated model already" do
+      product_decorator = ProductDecorator.new(source)
+      ProductDecorator.new(product_decorator).model.should be_instance_of Product
+    end
+
     it "should handle plural-like words properly'" do
       class Business; end
       expect do