Prevent crash for servers that don't send etags with repo indexes

While working on #711 (closed), I noticed this bug using my Mock HTTP Auth server.

This fix always captures timestamps during repo updates, even if it is the same as last time. This is because we are dependent on it later on in the repo update process. Specifically, when updating from a HTTP server that doesn't send out etags with its responses, it will trigger a full blown repo update every time, even if all the values in the index are the same (name, description, etc). This is as distinct from better behaving servers that send etags, in which case we will only do a partial update (i.e. persist the "last updated time"). In such a case, the remainder of the update process will proceed, and ask for this timestamp.