Fix `can't modify frozen String` error in `DebugLocks`

Without this, `DebugLocks` middleware raises an error as follwing:

```
Puma caught this error: can't modify frozen String (RuntimeError)
actionpack/lib/action_dispatch/middleware/debug_locks.rb:97:in `block in render_details'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `each'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `map'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `render_details'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:37:in `call'
railties/lib/rails/engine.rb:524:in `call'
```

1 files changed, 38 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/debug_locks_test.rb b/actionpack/test/dispatch/debug_locks_test.rb
new file mode 100644
index 0000000000..d69614bd79
--- /dev/null
+++ b/actionpack/test/dispatch/debug_locks_test.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+
+class DebugLocksTest < ActionDispatch::IntegrationTest
+  setup do
+    build_app
+  end
+
+  def test_render_threads_status
+    thread_ready = Concurrent::CountDownLatch.new
+    test_terminated = Concurrent::CountDownLatch.new
+
+    thread = Thread.new do
+      ActiveSupport::Dependencies.interlock.running do
+        thread_ready.count_down
+        test_terminated.wait
+      end
+    end
+
+    thread_ready.wait
+
+    get "/rails/locks"
+
+    test_terminated.count_down
+
+    assert_match(/Thread.*?Sharing/, @response.body)
+  ensure
+    thread.join
+  end
+
+  private
+    def build_app
+      @app = self.class.build_app do |middleware|
+        middleware.use ActionDispatch::DebugLocks
+      end
+    end
+end