rails3: fix our IE iframe upload workaround

In rails3, `render :text` was still returning a content-type of
application/json, presumably because of the request headers, which means
that IE was once again prompting users to download the api json response
when doing file uploads in a hidden iframe.

This commit forces the content-type to text/html to match the rails2
behavior, and adds a missing spec.

fixes CNVS-14412

test plan:

do a QTI course import using IE 10/11, and make sure you don't get
prompted to download a "create_success" file when the zip upload
finishes.

Change-Id: I5fce6885ab8bf321163439c8ed1c5e274c4f9d7a
Reviewed-on: https://gerrit.instructure.com/38506
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>