Backport ruby/ruby@09d8c99 (#91)

/cc @byroot

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

Author: hsbt

test/stringio/test_ractor.rb

@@ -11,7 +11,7 @@ def test_ractor
       require "stringio"
       $VERBOSE = nil
       r = Ractor.new do
-        io = StringIO.new("")
+        io = StringIO.new(+"")
         io.puts "abc"
         io.truncate(0)
         io.puts "def"