Incorrect Vary and etag for desktop indicies

[Michael Nelson]

Requests to /api/2.0/applications/en/ubuntu/trusty/amd64/ and similar urls, used by the desktop USC, and being rendered with a bunch of Vary headers which should not be present (the URI is unauthenticated, does not vary on language, afaict) [1]

The etag returned is also incorrect, in the sense that it should be the md5 of the uncompressed content appended with ';gzip', as per the django source code [2], whereas what we get is the md5 of the compressed content, which is different for identical uncompressed content due to gzip including a timestamp header (see [1]). The effect of this is that all re-validation of content based on etags fails for these URIs.

[1] https://pastebin.canonical.com/130410/
[2] https://github.com/django/django/blob/master/django/middleware/gzip.py#L43