fix: preserve all Set-Cookie headers in webob_to_django_response

[cpruijsen]

Only the last Set-Cookie header survives the WebOb to Django conversion. webob_to_django_response
in xblock/django/request.py assigns each header with django_response[name] = value, and a Django
response's header mapping holds one value per name, so each cookie overwrites the previous one. A
handler that sets several cookies gets one.

Set-Cookie is the header where this matters, because it is the one servers legitimately send more
than once. This routes those headers into the response's cookie jar with cookies.load(value), which
is the structure Django serialises each cookie from, and leaves every other header on the existing
path.

Changelog entry included, per the repo convention.

Fixes #523

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