fix Multipart::Post Content-type header

This reverts commit e3148346df900ac4e38a9ad643266dde26fe7515
This reverts commit 16f518a1305fac6c031f3bc8ca2b2d012d952e61
This partially reverts 9d34baed66b7856205230992e72d05db58e9e186

fixes CNVS-12021

test plan (e3148346df900ac4e38a9ad643266dde26fe7515)
 - rails 3 specs should pass
test plan (9d34baed66b7856205230992e72d05db58e9e186)
 - import content including a video file into a course
      (such as the QTI file attached to CNVS-11602)
 - the import should complete
test plan (16f518a1305fac6c031f3bc8ca2b2d012d952e61)
 - submit a document to turnitin
 - it should successfully receive a score

Change-Id: I0622d9eceee4b94e488f103db68290f1b641b555
Reviewed-on: https://gerrit.instructure.com/32333
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>