Only return attributes implemented by the decorator

Closes #444

lib/draper/decorator.rb

@@ -187,8 +187,14 @@ module Draper
       self
     end
 
+    # @return [Hash] the source's attributes, sliced to only include those
+    # implemented by the decorator.
+    def attributes
+      source.attributes.select {|attribute, _| respond_to?(attribute) }
+    end
+
     # ActiveModel compatibility
-    delegate :attributes, :to_param, :to_partial_path
+    delegate :to_param, :to_partial_path
 
     # ActiveModel compatibility
     singleton_class.delegate :model_name, to: :source_class

spec/draper/decorator_spec.rb

@@ -345,6 +345,15 @@ module Draper
       end
     end
 
+    describe "#attributes" do
+      it "returns only the source's attributes that are implemented by the decorator" do
+        decorator = Decorator.new(double(attributes: {foo: "bar", baz: "qux"}))
+        decorator.stub(:foo)
+
+        expect(decorator.attributes).to eq({foo: "bar"})
+      end
+    end
+
     describe ".model_name" do
       it "delegates to the source class" do
         Decorator.stub source_class: double(model_name: :delegated)