Force binary string in wim parser expectation

Force binary in an expectation since UTF-8 is the default in ruby 2.0. …
When reading ruby file contents, ruby 2+ encodes any Strings as UTF-8.
We have to force the encoding since WimParser returns either FixNums or ASCII-8BIT Strings.

See: https://www.ruby-lang.org/en/news/2013/02/24/ruby-2-0-0-p0-is-released/

Note: This is backward compatible with ruby 1.9.3.