encoding/asn1: fix explicitly tagged Times.

https://codereview.appspot.com/153770043/ tried to fix the case where a
implicitly tagged Time, that happened to have the same tag as
GENERALIZEDTIME, shouldn't be parsed as a GENERALIZEDTIME.

It did so, mistakenly, by testing whether params.tag != nil. But
explicitly tagged values also have a non-nil tag and there the inner
tag actually does encode the type of the value.

This change instead tests whether the tag class is UNIVERSAL before
assuming that the tag contains type information.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/152380044