Fixes #5376 DateTime to minutes

Due to the way DateTime objects are converted to a number of minutes from now, it's possible to unintentionally cache something forever.

Example: the following code will put a value into the cache that lasts forever, but the code reads as if it should expire in 1 minute. In real use-cases, sleep(60) would be effected by a slow running database query, some slow network operation, or whatever.

    $expiresAt = Carbon::now()->addMinutes(1);

    $value = Cache::remember('users', $expiresAt, function()
    {
        sleep(60);
        return true;
    });

This PR changes the behaviour so that the value is simply not cached in this situation.