Fix `can't modify frozen String` error in AC::Rendering

2 files changed, 13 insertions, 1 deletions

diff --git a/actionpack/lib/action_controller/metal/rendering.rb b/actionpack/lib/action_controller/metal/rendering.rb
index d32eabf9ba..dcb48be80a 100644
--- a/actionpack/lib/action_controller/metal/rendering.rb
+++ b/actionpack/lib/action_controller/metal/rendering.rb
@@ -42,7 +42,7 @@ module ActionController
     def render_to_string(*)
       result = super
       if result.respond_to?(:each)
-        string = ""
+        string = "".dup
         result.each { |r| string << r }
         string
       else
diff --git a/actionpack/test/controller/render_test.rb b/actionpack/test/controller/render_test.rb
index 3619afc513..1ca668b9b1 100644
--- a/actionpack/test/controller/render_test.rb
+++ b/actionpack/test/controller/render_test.rb
@@ -606,6 +606,18 @@ class MetalRenderTest < ActionController::TestCase
   end
 end
 
+class ActionControllerRenderTest < ActionController::TestCase
+  class MinimalController < ActionController::Metal
+    include AbstractController::Rendering
+    include ActionController::Rendering
+  end
+
+  def test_direct_render_to_string_with_body
+    mc = MinimalController.new
+    assert_equal "Hello world!", mc.render_to_string(body: ["Hello world!"])
+  end
+end
+
 class ActionControllerBaseRenderTest < ActionController::TestCase
   def test_direct_render_to_string
     ac = ActionController::Base.new()