fix Content-type for turnitin post requests

fixes CNVS-12011

test plan:
- submit a document to turnitin
- it should successfully receive a turnitin score

Change-Id: I4b4459e134ae8897896ed4a6200917d7fed1d45f
Reviewed-on: https://gerrit.instructure.com/32344
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>