encoding/asn1: respect “explicit” and “tag” when unmarshaling RawValues.

encoding/asn1: respect “explicit” and “tag” when unmarshaling RawValues.

Previously, any “explicit” and/or “tag” decorations on a RawValue would
be ignored when unmarshaling. The RawValue would swallow whatever
element was encountered.

This change causes these decorations to be respected. Thus a field like:
  Foo asn1.RawValue `asn1:"explicit,tag:1,optional"`
will only match if an explicit tag with value one is encountered.
Otherwise the RawValue will get the default value and parsing will move
onto the next element.

Thanks to Martin Kreichgauer for reporting the issue.

Change-Id: If6c4488685b9bd039cb5e352d6d75744f98dbb1f
Reviewed-on: https://go-review.googlesource.com/34503
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>