Fix failing specs re Rails 5.1 (#800)

* Fix failing specs re Rails 5.1

Because there is no hard constraint on Rails version, tests run against
the newest version available. Rails 5.1 removes `to_h` from unpermitted
parameters class. Using `empty?` is sufficient replacement.

* Fix Dummy app `posts#mail` action rendering

Rails 5.1 does not accept `:text` key for `render`.

spec/draper/view_context/build_strategy_spec.rb

@@ -38,7 +38,7 @@ module Draper
         expect(controller.request).to be_nil
         strategy.call
         expect(controller.request).to be_an ActionController::TestRequest
-        expect(controller.params.to_h).to eq({})
+        expect(controller.params).to be_empty
 
         # sanity checks
         expect(controller.view_context.request).to be controller.request

spec/dummy/app/controllers/posts_controller.rb

@@ -8,7 +8,7 @@ class PostsController < BaseController
   def mail
     post = Post.find(params[:id])
     email = PostMailer.decorated_email(post).deliver
-    render text: email.body
+    render html: email.body.to_s.html_safe
   end
 
   private