This was how I solved mocking the request object for a Draper decorator from RSpec.

article_decorator_spec.rb

describe 'ArticleDecorator'
  describe '#opengraph_image_url' do
    it 'should provide the default image as a fallback' do
      request = stub(host: 'local.example.com', port: 3000)
      ArticleDecorator.any_instance.stub request: request
      ArticleDecorator.new(Article.new).opengraph_image_url.should == "http://local.example.com:3000/images/default.png"
    end
  end
end