Set encoding to 'BINARY' when creating a StringIO - this will keep from ...

This sets the encoding of the created StringIO (when doing download! to StringIO) to 'BINARY'.
Without this change, I was seeing weird errors downloading a zip file.   The file is 7402199 bytes, and doing a progress on the download showed it downloaded it all, but the returned StringIO's #length returned 7132662.
When I downloaded to a file instead, the file had the expected length of 7402199.   I did some searching and found http://blog.rayapps.com/2013/03/11/7-things-that-can-go-wrong-with-ruby-19-string-encodings/ (see #5 Sending binary data with default UTF-8 encoding).  Based on that, I modified net-scp like this and I was able to download my file properly.