ASN.1 elements can be optional, and can have a default value.
Traditionally, Go has omitted elements that are optional and that have
the zero value. I believe that's a bug (see [1]).

This change causes an optional element with a default value to only be
omitted when it has that default value. The previous behaviour of
omitting optional, zero elements with no default is retained because
it's used quite a lot and will break things if changed.

[1] https://groups.google.com/d/msg/Golang-nuts/9Ss6o9CW-Yo/KL_V7hFlyOAJ

Fixes #7780.

R=bradfitz

LGTM=bradfitz
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews, r
https://golang.org/cl/86960045