Added #decorated? method to Draper::CollectionDecorator

Closes #447

lib/draper/collection_decorator.rb

@@ -70,6 +70,11 @@ def decorator_class
       @decorator_class ||= self.class.inferred_decorator_class
     end
 
+    # @return [true]
+    def decorated?
+      true
+    end
+
     protected
 
     # @return the collection being decorated.

spec/draper/collection_decorator_spec.rb

@@ -255,5 +255,13 @@ module Draper
       end
     end
 
+    describe '#decorated?' do
+      it 'returns true' do
+        decorator = ProductsDecorator.new([Product.new])
+
+        expect(decorator).to be_decorated
+      end
+    end
+
   end
 end