goprotobuf: Rename some fields for clarity.

R=r
CC=golang-dev
http://codereview.appspot.com/6576047
diff --git a/proto/lib.go b/proto/lib.go
index 28f9fff..4b3f48e 100644
--- a/proto/lib.go
+++ b/proto/lib.go
@@ -697,7 +697,7 @@
 func buildDefaultMessage(t reflect.Type) (dm defaultMessage) {
 	sprop := GetProperties(t)
 	for _, prop := range sprop.Prop {
-		fi, ok := sprop.tags.get(prop.Tag)
+		fi, ok := sprop.decoderTags.get(prop.Tag)
 		if !ok {
 			// XXX_unrecognized
 			continue