You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: test_stringio.rb
    path of file: ./repos/ruby/test/stringio
    
    the code of the file until where you have to start completion: # frozen_string_litera
    
  def test_initialize
    assert_kind_of StringIO, StringIO.new
    assert_kind_of StringIO, StringIO.new('str')
    assert_kind_of StringIO, StringIO.new('str', 'r+')
    assert_raise(ArgumentError) { StringIO.new('', 'x') }
    assert_raise(ArgumentError) { StringIO.new('', 'rx') }
    assert_raise(ArgumentError) { StringIO.new('', 'rbt') }
    assert_raise(TypeError) { StringIO.new(nil) }

    o = Object.new
    def o.to_str
      nil
    end
    assert_raise(TypeError) { StringIO.new(o) }

    o = Object.new
    def o.to_str
      'str'
    end
    assert_kind_of StringIO, StringIO.new(o)
  end