fix: issue correct expiry dates for tokens (#2)

related to a NodeJS (nodejs/node#7074) and furthermore
V8 bug (https://bugs.chromium.org/p/v8/issues/detail?id=3637); replaced
seconds calculation with milliseconds.

Author: razvanz

lib/grant-types/abstract-grant-type.js

@@ -67,23 +67,15 @@ AbstractGrantType.prototype.generateRefreshToken = function(client, user, scope)
  */
 
 AbstractGrantType.prototype.getAccessTokenExpiresAt = function() {
-  var expires = new Date();
-
-  expires.setSeconds(expires.getSeconds() + this.accessTokenLifetime);
-
-  return expires;
+  return new Date(Date.now() + this.accessTokenLifetime * 1000);
 };
 
 /**
  * Get refresh token expiration date.
  */
 
 AbstractGrantType.prototype.getRefreshTokenExpiresAt = function() {
-  var expires = new Date();
-
-  expires.setSeconds(expires.getSeconds() + this.refreshTokenLifetime);
-
-  return expires;
+  return new Date(Date.now() + this.refreshTokenLifetime * 1000);
 };
 
 /**