This change fixes #530, Net::SSH failure.

In the same way that UTF-8 sucks in all ASCII compatible strings and the result is UTF-8, any string that is ASCII-8BIT (BINARY, internally) will suck in anything that is combined with it and the result will be ASCII-8BIT (BINARY).

Funny thing is I came up with a reduction that fails in macruby and 1.9.3, but Net:SSH fails in macuby and works in 1.9.3!  Go figure.