expose ETag in submission/reply downloads

[redshiftzero]

(i figured I'd do this one so that we can make a new release of securedrop-sdk after this is merged with the other change we want for this sprint: #95)

Closes #86

You can test interactively, or you can also just inspect the tests and existing test cassettes since the ETags headers are there in the requests/responses

[rmol]

👍 Tests passed. Script outside of the test suite worked too, for both submissions and replies.

My only comment is that the HTTP header is "ETag", not "Etag". That's the way we're sending it from the server, and it matters not at all because Requests' header dict is case-insensitive, but I just can't help myself. 😞 I may have to file an issue for securedrop. 🙂

[redshiftzero]

thanks for reviewing! your comment re: Etag case is a good flag, fortunately since in HTTP/1.1 header names are case insensitive as long as we are not expecting a certain case of the ETag header elsewhere in our codebase we shouldn't need to make any changes