Fix gallery has media API methods to fill position/enabled fields

I've fixed the following methods to give the opportunity to fill the `position` and the `enabled` fields when adding or updating a media in a gallery:

![capture decran 2014-03-18 a 12 03 52](https://f.cloud.github.com/assets/103900/2446905/4cc8f858-ae8d-11e3-96a8-8206f311896f.png)