The existing implementation is erroneously assume that having no
deadline in context.Context means that time returned from Deadline
method will have IsZero() == true. But technically speaking this is an
invalid assumption. The context.Context interface specification doesn't
specify what time should be returned from Deadline method when there is
no deadline set. It only specifies that second result of Deadline should
be false.

Fixes #35594

Change-Id: Ife00aad77ab3585e469f15017550ac6c0431b140
Reviewed-on: https://go-review.googlesource.com/c/go/+/207297
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>