Escape characters < 0x20 in strings

[ohler55]

Make sure the parser when strict warns about characters less than a space. These provided by @byroot are:

 def test_unescaped_ASCII_control_characters
    p = Oj::Parser.new(:usual)

    (0..31).each do |control_ord|
      assert_raises(Oj::ParseError) do
        p.parse(%{"invalid character: #{control_ord.chr}"})
      end
    end
  end

def test_unescaped_ASCII_control_characters
    (0..31).each do |control_ord|
      assert_raises(Oj::ParseError) do
        Oj.load(%{"invalid character: #{control_ord.chr}"})
      end
    end
  end