Fix error message from KeyForObj when passing an invalid struct.

filter/dscache/support.go

Index: filter/dscache/support.go
diff --git a/filter/dscache/support.go b/filter/dscache/support.go
index ea64460886d971efa1272e9e7072d08e3058f60c..cc0a8f246eaaf36eff7072b459abe0cc16815efd 100644
--- a/filter/dscache/support.go
+++ b/filter/dscache/support.go
@@ -42,7 +42,13 @@ func (s *supportContext) numShards(k *ds.Key) int {
 func (s *supportContext) mkRandKeys(keys []*ds.Key, metas ds.MultiMetaGetter) []string {
 	ret := []string(nil)
 	for i, key := range keys {
-		if !metas.GetMetaDefault(i, CacheEnableMeta, true).(bool) {
+		mg := metas.GetSingle(i)
+		v, err := ds.GetMetaDefault(mg, CacheEnableMeta, true)
+		if err != nil {
+			// TODO(riannucci): Not sure what to do with this
+			panic(err)
+		}
+		if !v.(bool) {
 			continue
 		}
 		shards := s.numShards(key)