Use `until` instead of `at` query parameter for server

[jbyler]

Bitbucket Server web UI uses until parameter, not at, for the cases I could find to test. Both seem to work, so I'm not sure if this change is important.

[mark2185]

I'd leave this open so we have it at hand in case the current approach does break at some point, but I wouldn't merge it if nobody has complained so far.