Draper.undecorate safely undecorates any object

This allows you to pass any object to Draper.undecorate knowing you will get the original object back
even if the object passed in does is not already decorated.

Closes #600

lib/draper.rb

@@ -22,6 +22,7 @@ require 'draper/decorated_association'
 require 'draper/helper_support'
 require 'draper/view_context'
 require 'draper/collection_decorator'
+require 'draper/undecorate'
 require 'draper/decorates_assigned'
 require 'draper/railtie' if defined?(Rails)
 

lib/draper/undecorate.rb

@@ -0,0 +1,9 @@
+module Draper
+  def self.undecorate(object)
+    if object.respond_to?(:decorated?) && object.decorated?
+      object.object
+    else
+      object
+    end
+  end
+end

spec/draper/undecorate_spec.rb

@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe Draper, '.undecorate' do
+  it 'undecorates a decorated object' do
+    object = Model.new
+    decorator = Draper::Decorator.new(object)
+    expect(Draper.undecorate(decorator)).to equal object
+  end
+
+  it 'passes a non-decorated object through' do
+    object = Model.new
+    expect(Draper.undecorate(object)).to equal object
+  end
+
+  it 'passes a non-decorator object through' do
+    object = Object.new
+    expect(Draper.undecorate(object)).to equal object
+  end
+end