Bug 1054472: Replace/simplify ETag code.

[Comment 5 in Bugzilla](https://bugzilla.mozilla.org/show_bug.cgi?id=1054472#c5) explains why the previous logic for returning a 304 in response to matching ETags makes no difference with Zeus in play. Instead, we want to just add an ETag header to the response, which is much simpler!
OrderedDict was included in python 2.7 :( That's why tests fail.