+ keep 'getWithSnapshot' consistent with levigo implementation

- this change keeps the method behavior consistent with the
     levigo/leveldb implementation.

   - the leveldb store_test.go and goleveldb store_test.go are now
     identical.

index/store/goleveldb/store.go

@@ -55,6 +55,9 @@ func (ldbs *Store) get(key []byte) ([]byte, error) {
 func (ldbs *Store) getWithSnapshot(key []byte, snapshot *leveldb.Snapshot) ([]byte, error) {
 	options := defaultReadOptions()
 	b, err := snapshot.Get(key, options)
+	if err == leveldb.ErrNotFound {
+		return nil, nil
+	}
 	return b, err
 }
 

index/store/goleveldb/store_test.go

@@ -204,7 +204,7 @@ func CommonTestReaderIsolation(t *testing.T, s store.KVStore) {
 
 	// but that the isolated reader does not
 	val, err = reader.Get([]byte("b"))
-	if err != nil && err.Error() != "leveldb: not found" {
+	if err != nil {
 		t.Error(err)
 	}
 	if val != nil {