Fix unexpected nil pointer when reading iterm from keyring (#470)

diff --git a/oauth2/store/keyring.go b/oauth2/store/keyring.go
index fd49baf..8a024f5 100644
--- a/oauth2/store/keyring.go
+++ b/oauth2/store/keyring.go
@@ -117,7 +117,7 @@
 	defer f.lock.Unlock()
 
 	key := hashKeyringKey(audience)
-	authItem, err := f.kr.GetMetadata(key)
+	authItem, err := f.kr.Get(key)
 	if err != nil {
 		if err == keyring.ErrKeyNotFound {
 			return "", ErrNoAuthenticationData