test edge case

lib/KeyStore.ts

@@ -11,6 +11,10 @@ class KeyStore {
       return this._keys[key];
 
     if (valid_for !== 0) {
+      if ((iat + valid_for) * 1000 < (new Date)
+        .getTime ())
+        throw new Error ('cannot create already expired keys');
+
       this._keys[key] = create_salt ();
       setTimeout (() => {
         delete this._keys[key];