fixing missing 'position' property notice while iterating over article tags

At godt.no we're getting a lot of notices "Undefined property: stdClass::$position in ­vendor/­aptoma/­drpublish-api-client/­lib/­core/­entities/­DrPublishApiClientArticle.php:241". After investigating DrPublish API response, e.g. http://dp-api.vg.no/articles/20355197.json, you can see that there is no such attribute 'position'.

I prepared a PR which returns first tag as the MainDPTag.