VapidExpiration option is unused

[sometimes-i-send-pull-requests]

webpush.Options has a dedicated VapidExpiration field documented as:

	VapidExpiration time.Time  // optional expiration for VAPID JWT token (defaults to now + 12 hours)

It was introduced in #45. SendNotificationWithContext passes it into getVAPIDAuthorizationHeader, and in #45, getVAPIDAuthorizationHeader used it when forming the exp claim. However, what looks to have been a faulty merge conflict resolution in #46 seems to have reverted the change in getVAPIDAuthorizationHeader, leaving expiration unused:

 	token := jwt.NewWithClaims(jwt.SigningMethodES256, jwt.MapClaims{
-		"aud": fmt.Sprintf("%s://%s", subURL.Scheme, subURL.Host),
-		"exp": expiration.Unix(),
-		"sub": fmt.Sprintf("mailto:%s", subscriber),
+		"aud": subURL.Scheme + "://" + subURL.Host,
+		"exp": time.Now().Add(time.Hour * 12).Unix(),
+		"sub": "mailto:" + subscriber,
 	})

As a result, the VAPID JWT expiration is always now+12 hours, even when explicitly set otherwise.