I'm writing a matching implementation of the time package and missed
the "add one day in a leap year" block. This test would have caught my
error.

I understand we can't add test cases for every Date but it seems like
"tripped up someone attempting to reimplement this" is a good
indicator it may trip up people in the future.

Change-Id: I4c3b51e52e269215ec0e52199afe604482326edb
Reviewed-on: https://go-review.googlesource.com/56490Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>