RateLimit-Reset header from RackAttack differs from that returned by haproxy

The RateLimit-Reset header from haproxy is %[date(60)], which is the end of the current minute in unix epoch time (e.g. 1610590696); the value from RackAttack, only recently added, is the same as that for Retry-After, which is the number of seconds until the client should retry (i.e. when the current RateLimit period ends).

I think this was inadvertent, and we should modify RackAttack to return the epoch time instead.

/cc @qmnguyen0711