rack SPEC doesn't allow `first` on the body

collect the body and make assert against the collected body

actionpack/test/controller/new_base/bare_metal_test.rb

@@ -37,8 +37,6 @@ class BareTest < ActiveSupport::TestCase
       controller = BareController.new
       controller.set_request! ActionDispatch::Request.new(env)
       assert controller.request
-      assert controller.response
-      assert env['action_controller.instance']
     end
   end
 
@@ -123,34 +121,40 @@ class HeadTest < ActiveSupport::TestCase
     end
 
     test "head :no_content (204) does not return any content" do
-      content = HeadController.action(:no_content).call(Rack::MockRequest.env_for("/")).third.first
+      content = body(HeadController.action(:no_content).call(Rack::MockRequest.env_for("/")))
       assert_empty content
     end
 
     test "head :reset_content (205) does not return any content" do
-      content = HeadController.action(:reset_content).call(Rack::MockRequest.env_for("/")).third.first
+      content = body(HeadController.action(:reset_content).call(Rack::MockRequest.env_for("/")))
       assert_empty content
     end
 
     test "head :not_modified (304) does not return any content" do
-      content = HeadController.action(:not_modified).call(Rack::MockRequest.env_for("/")).third.first
+      content = body(HeadController.action(:not_modified).call(Rack::MockRequest.env_for("/")))
       assert_empty content
     end
 
     test "head :continue (100) does not return any content" do
-      content = HeadController.action(:continue).call(Rack::MockRequest.env_for("/")).third.first
+      content = body(HeadController.action(:continue).call(Rack::MockRequest.env_for("/")))
       assert_empty content
     end
 
     test "head :switching_protocols (101) does not return any content" do
-      content = HeadController.action(:switching_protocols).call(Rack::MockRequest.env_for("/")).third.first
+      content = body(HeadController.action(:switching_protocols).call(Rack::MockRequest.env_for("/")))
       assert_empty content
     end
 
     test "head :processing (102) does not return any content" do
-      content = HeadController.action(:processing).call(Rack::MockRequest.env_for("/")).third.first
+      content = body(HeadController.action(:processing).call(Rack::MockRequest.env_for("/")))
       assert_empty content
     end
+
+    def body(rack_response)
+      buf = []
+      rack_response[2].each { |x| buf << x }
+      buf.join
+    end
   end
 
   class BareControllerTest < ActionController::TestCase