datastore: use proper struct tag when querying properties in KindProperties

With the current struct tag this function always returns the following error:
datastore: cannot load field "property_representation" into a "struct { Repr []string \"datastore:property_representation\" }": no such struct field

Change-Id: Icd83f6cd2c072c7fa819941c01ae119b8e8c0e05
diff --git a/datastore/metadata.go b/datastore/metadata.go
index 67995f9..6acacc3 100644
--- a/datastore/metadata.go
+++ b/datastore/metadata.go
@@ -64,7 +64,7 @@
 
 	propMap := map[string][]string{}
 	props := []struct {
-		Repr []string `datastore:property_representation`
+		Repr []string `datastore:"property_representation"`
 	}{}
 
 	keys, err := q.GetAll(ctx, &props)