internal/value: fix GoValueOf for v2 message types
internal/value: fix GoValueOf for v2 message types

Was using a pref.Message where we want a pref.ProtoMessage.

Change-Id: I61d986a43eaf8f945a1378a7a10120474aa89d6f
Reviewed-on: https://go-review.googlesource.com/c/153697
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go