rate: use time.Time.Equal instead of ==

According to the Time Type documentation, in most cases, it is better
to use Equal instead of ==


Change-Id: I201de68ee163a06cf9ed49f7b77a2de031dbdd95
Reviewed-on: https://go-review.googlesource.com/c/time/+/703175
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>