Only last cookie is sent to client in response

When setting multiple headers with the same name Set-Cookie, only the last one is actually in the django response object.
For instance, this response:

HTTP/2.0 200 OK
Content-Type: text/html
Set-Cookie: yummy_cookie=choco
Set-Cookie: tasty_cookie=strawberry
[page content]

It will only set this Set-Cookie: tasty_cookie=strawberry to response.

A call to django_response.set_cookie() method if the header is "Set-Cookie" might fix it.

https://github.com/edx/XBlock/blob/316cc376945f0c748d4ec7b780ee55bd6aef4425/xblock/django/request.py#L12

https://tools.ietf.org/html/rfc7230

Note: In practice, the "Set-Cookie" header field ([RFC6265]) often
appears multiple times in a response message and does not use the
list syntax, violating the above requirements on multiple header
fields with the same name. Since it cannot be combined into a
single field-value, recipients ought to handle "Set-Cookie" as a
special case while processing header fields. (See Appendix A.2.3
of [Kri2001] for details.)

Start in xblock/django/request.py at the linked line and inspect how repeated Set-Cookie headers are transferred into the Django response object. Verify the response preserves both cookie values and that the resulting client response contains both Set-Cookie fields; no test file is named in the issue.