[ruby/tempfile] Make Tempfile#open return the underlying File

Add test for this behavior.

https://github.com/ruby/tempfile/commit/0ca31a6b8d

lib/tempfile.rb

@@ -191,6 +191,7 @@ class Tempfile < DelegateClass(File)
     mode = @mode & ~(File::CREAT|File::EXCL)
     __setobj__(File.open(__getobj__.path, mode, **@opts))
     ObjectSpace.define_finalizer(self, Closer.new(__getobj__))
+    __getobj__
   end
 
   def _close    # :nodoc:

test/test_tempfile.rb

@@ -378,6 +378,14 @@ puts Tempfile.new('foo').path
     assert_file.not_exist?(path)
   end
 
+  def test_open
+    Tempfile.open {|f|
+      file = f.open
+      assert_kind_of File, file
+      assert_equal f.to_i, file.to_i
+    }
+  end
+
   def test_open_traversal_dir
     assert_mktmpdir_traversal do |traversal_path|
       t = Tempfile.open([traversal_path, 'foo'])