Returns a copy of +self+ with each invalid byte sequence replaced
by the given +replacement_string+.

With no block given and no argument, replaces each invalid sequence
with the default replacement string
(<tt>"�"</tt> for a Unicode encoding, <tt>'?'</tt> otherwise):

  s = "foo\x81\x81bar"
  s.scrub # => "foo��bar"

With no block given and argument +replacement_string+ given,
replaces each invalid sequence with that string:

  "foo\x81\x81bar".scrub('xyzzy') # => "fooxyzzyxyzzybar"

With a block given, replaces each invalid sequence with the value
of the block:

  "foo\x81\x81bar".scrub {|bytes| p bytes; 'XYZZY' }
  # => "fooXYZZYXYZZYbar"

Output:

  "\x81"
  "\x81"
