make ZipExtractor write files in binary mode

test plan:
 - use rails3
 - create an assignment that accepts file submissions
 - as a student, submit something that isn't plaintext (such as
   an Office document; anything that isn't plain UTF-8 will work)
 - as a teacher, go to the assignment page and select
   "Download Submissions"
 - then refresh the page and select "Re-Upload Submissions",
   and upload the file you just downloaded
 - you should not get a page error

fixes CNVS-12369

Change-Id: I676efc2e8c54419b6bb0ad274d42d0342241a82f
Reviewed-on: https://gerrit.instructure.com/33245
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>