Contribute: Add proper header for export course download

[OmarIthawi]

#559 seems like a good fix to contribute upstream.

[thraxil]

Agreed, although it would have to be part of a larger patch to add boto3 support since they're still using the old version: https://github.com/edx/edx-platform/blob/master/cms/djangoapps/contentstore/views/import_export.py#L376

They are already on django-storages 1.8, so we don't need to include #548 but it will need #549 #550 and #559

[OmarIthawi]

Thanks Anders for the info.