Only tr once we've set the encoding to BINARY

String#tr fails if the input string contains invalid Unicode characters. This
is fine, but only if the string realizes that it's binary data rather than
Unicode encoded characters. So we force it to binary mode *before* we
String#tr.

The String#dup doesn't actually duplicate the string's contents, or we'd worry
about making a copy of the potentially-sensitive data in memory. This isn't an
issue for the exact reason we're doing String#tr in the first place: Ruby is
smart enough to copy-on-write the memory used for the String.