Description:
------------
Europe/Oslo has DST and it occurs at (got with `(new DateTimeZone("Europe/Oslo"))->getTransitions()`):

```
     [
       "ts" => 1521939600,
       "time" => "2018-03-25T01:00:00+0000",
       "offset" => 7200,
       "isdst" => true,
       "abbr" => "CEST",
     ],
     [
       "ts" => 1540688400,
       "time" => "2018-10-28T01:00:00+0000",
       "offset" => 3600,
       "isdst" => false,
       "abbr" => "CET",
     ],
```

The following test script increments the input date time to 1 hour which should not occur?. I'm not sure either this is a bug or not as I don't have enough information to decide but probably it can be the bug.

Test script:
---------------
<?php
// For the start date time + 1 hour it prints 2018-03-25 03:00:00, which is incorrect?
echo (new \DateTimeImmutable('2018-03-25 02:00:00', new \DateTimeZone('Europe/Oslo')))->format('Y-m-d H:i:s');

// However for the end date time it does not change the value, tried for the dates: "2018-10-27 23:00:00", "2018-10-28 00:00:00", "2018-10-28 01:00:00", "2018-10-28 02:00:00", "2018-10-28 03:00:00", "2018-10-28 04:00:00", "2018-10-28 05:00:00", "2018-10-28 06:00:00" like so:
// echo (new \DateTimeImmutable('2018-10-28 03:00:00', new \DateTimeZone('Europe/Oslo')))->format('Y-m-d H:i:s');
// It always returns the same value, which I think is correct.


Expected result:
----------------
2018-03-25 02:00:00

Actual result:
--------------
2018-03-25 03:00:00