move response allocation to the class level

we don't need an instance to figure out what type of response to
allocate.  Later we'll pull this up the stack and pass the response
object down

actionpack/lib/action_controller/metal/live.rb

@@ -33,6 +33,20 @@ module ActionController
   # the main thread. Make sure your actions are thread safe, and this shouldn't
   # be a problem (don't share state across threads, etc).
   module Live
+    extend ActiveSupport::Concern
+
+    module ClassMethods
+      def make_response!(request)
+        if request.env["HTTP_VERSION"] == "HTTP/1.0"
+          super
+        else
+          Live::Response.new.tap do |res|
+            res.request = request
+          end
+        end
+      end
+    end
+
     # This class provides the ability to write an SSE (Server Sent Event)
     # to an IO stream. The class is initialized with a stream and can be used
     # to either write a JSON string or an object which can be converted to JSON.
@@ -311,12 +325,7 @@ def response_body=(body)
     end
 
     def set_response!(request)
-      if request.env["HTTP_VERSION"] == "HTTP/1.0"
-        super
-      else
-        @_response         = Live::Response.new
-        @_response.request = request
-      end
+      @_response = self.class.make_response! request
     end
   end
 end

actionpack/lib/action_controller/metal/rack_delegation.rb

@@ -12,6 +12,12 @@ module ClassMethods
       def build_with_env(env = {}) #:nodoc:
         new.tap { |c| c.set_request! ActionDispatch::Request.new(env) }
       end
+
+      def make_response!(request)
+        ActionDispatch::Response.new.tap do |res|
+          res.request = request
+        end
+      end
     end
 
     def set_request!(request) #:nodoc:
@@ -31,8 +37,7 @@ def reset_session
     private
 
     def set_response!(request)
-      @_response         = ActionDispatch::Response.new
-      @_response.request = request
+      @_response = self.class.make_response! request
     end
   end
 end