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/16517Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>