time: Time.Before() should reuse t.sec() and u.sec()

[geraldss]

Looking at the source code for Time.After() and Time.Before(), Time.After() reuses t.sec() and u.sec(), whereas Time.Before() does not and often calls each method twice.

https://golang.org/src/time/time.go

[tpaschalis]

I'm on it, let me wrap up a PR for this.

[gopherbot]

Change https://golang.org/cl/217360 mentions this issue: time: fix time.Before to reuse t.sec()/u.sec() and not call them