def add(photo)
      response = JSON.parse(connection.post("/collections/#{id}/add", { photo_id: photo.id }).body)
      {
        photo_id:      response["photo"]["id"],
        collection_id: response["collection"]["id"],
        user_id:       response["user"]["id"],
        created_at:    response["created_at"]
      }
    end
def add(photo)
      response = JSON.parse(connection.post("/collections/#{id}/add", { photo_id: photo.id }).body)
      {
        photo_id:      response["photo"]["id"],
        collection_id: response["collection"]["id"],
        user_id:       response["user"]["id"],
        created_at:    response["created_at"]
      }
    end
Add a photo to the collection. If the photo is already in the collection,
 this action has no effect.
 @param [Unsplash::Photo] The photo to add.
 @return [Hash] Collected photo metadata.