Replaces each byte in +self+ with the bitwise XOR of that byte and the
corresponding byte in +other+; returns +self+:

  s = "\xF0"
  s.bitwise_xor!("\xCC") # => "\x3C"
  s                      # => "\x3C"

+other+ is converted to a string using +to_str+.
Raises +ArgumentError+ if the two strings have different byte sizes.
The encoding of +self+ is not changed.
