dir server responses should include a Date: header even when not responding 200

Catalyst noticed that when the Tor client fetches a consensus from moria1 but moria1 responds "302 Not modified", moria1 does not include a Date: header in that response. So a Tor client whose clock is wrong won't be told about it at this stage.

The fix is that all dir responses should include the Date: header, not just 200 responses.