rails3: fix posting to the sis import api without a content-type

fixes CNVS-14808

test plan:

Post a zip file to the sis import API, without specifying the content-type:

   curl --data-binary "@zip_file_.zip" -H "Authorization: ..." 'http://canvas.dev/api/v1/accounts/self/sis_imports.json?import_type=instructure_csv'

The post should succeed, and the CSVs inside the zip should SIS process without error.

Change-Id: Ib97c3ba7475543fd03089400df4747b644645ca7
Reviewed-on: https://gerrit.instructure.com/39410
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
Tested-by: Brian Palmer <brianp@instructure.com>