encoding/asn1: enforce use of short form lengths.
BER allows the sender to choose either short form or long form where
both are legal, but DER requires the minimal one be used. Enforce this
and add a test. Fix one test which was not minimally-encoded and another
which would not distinguish rejecting the input because the long form
length wasn't minimally-encoded from rejecting it because long form was
chosen when short form was allowed.
Change-Id: I1b56fcca594dcdeddea9378b4fab427cbe7cd26d
Reviewed-on: https://go-review.googlesource.com/16517
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>