Returns a copy of +self+ that has ASCII-8BIT encoding;
the underlying bytes are not modified:

  s = "\x99"
  s.encoding   # => #<Encoding:UTF-8>
  t = s.b      # => "\x99"
  t.encoding   # => #<Encoding:ASCII-8BIT>

  s = "\u4095" # => "䂕"
  s.encoding   # => #<Encoding:UTF-8>
  s.bytes      # => [228, 130, 149]
  t = s.b      # => "\xE4\x82\x95"
  t.encoding   # => #<Encoding:ASCII-8BIT>
  t.bytes      # => [228, 130, 149]
