Fix bug with multiple audience values

This commit resolves a bug concerning multiple audience claims. The audience array was previously not being expanded before calling the `stringify` method. Because of this, the string cast failed (inside stringify) and thus no audience values were returned.

jwt/claims.go

diff --git a/jwt/claims.go b/jwt/claims.go
index 2f4f2e1..cc135d8 100644
--- a/jwt/claims.go
+++ b/jwt/claims.go
@@ -118,7 +118,9 @@
 		return []string{t}, true
 	case []string:
 		return t, true
-	case interface{}, []interface{}:
+	case []interface{}:
+		return stringify(t...)
+	case interface{}:
 		return stringify(t)
 	}
 	return nil, false