Fix Ruby 2.0.0 compatibility

In Ruby 2.0.0, the default encoding of source files was changed to UTF-8 and now, there are test suite errors such as:

    Failure/Error: stream.should == "\0\x20\0\xD0"
     ArgumentError:
       invalid byte sequence in UTF-8

Since the stream is obviously binary, it should be compared to binary data, hence use the #force_encoding("BINARY") to explicitly set the encoding of compared string.