This change corrects the serialization of asn1.Flag values, so that
when set, they serialize to an empty value, and when unset, they are
omitted. It also adds a format parameter that allows calling code
to control whether time.Time values are serialized as UTCTime or
GeneralizedTime.

Change-Id: I6d97abf009ea317338dab30c80f35a2de7e07104
Reviewed-on: https://go-review.googlesource.com/5970Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>