When using FakeDb, and I set the full path of a DbItem to go under the media library, the path gets changed to the /sitecore/content/home/image-item. 

public void MediaItemPath()
        {
            Sitecore.Data.ID mediaItemId = Sitecore.Data.ID.NewID;

            // create some media item. Location, fields and template are not important
            using (Sitecore.FakeDb.Db db = new Sitecore.FakeDb.Db())
            {
                var mediaFakeItem = new Sitecore.FakeDb.DbItem("my-image", mediaItemId);
                mediaFakeItem.FullPath = "/sitecore/media library/my-image";
                db.Add(mediaFakeItem);
                Sitecore.Data.Items.Item mediaItem = db.GetItem(mediaItemId);

                Assert.AreEqual("/sitecore/media library/my-image", mediaFakeItem.FullPath);

            }
        }

How can I add an item to the media library?