Prevent [response].flatten from recursing infinitely.

Returning `self` from within the array returned by `to_ary`
caused this. Instead, we can just substitute another object.
It provides the `each` behavior required by the rack spec.

actionpack/lib/action_dispatch/http/response.rb

@@ -313,7 +313,7 @@ module ActionDispatch # :nodoc:
         header.delete CONTENT_TYPE
         [status, header, []]
       else
-        [status, header, self]
+        [status, header, Rack::BodyProxy.new(self){}]
       end
     end
   end

actionpack/test/dispatch/response_test.rb

@@ -226,6 +226,15 @@ class ResponseTest < ActiveSupport::TestCase
     assert_equal({ 'Content-Type' => 'text/plain' }, headers)
     assert_equal ['Not Found'], body.each.to_a
   end
+
+  test "[response].flatten does not recurse infinitely" do
+    Timeout.timeout(1) do # use a timeout to prevent it stalling indefinitely
+      status, headers, body = [@response].flatten
+      assert_equal @response.status, status
+      assert_equal @response.headers, headers
+      assert_equal @response.body, body.each.to_a.join
+    end
+  end
 end
 
 class ResponseIntegrationTest < ActionDispatch::IntegrationTest