def add_sites(id, sites)
      sites = Array(sites)
      payload = { 'site_ids' => sites }.to_json
      current_path = "/api/v1/collections/#{id}/add"
      @conn.post(current_path, payload)
    end
def add_sites(id, sites)
      sites = Array(sites)
      payload = { 'site_ids' => sites }.to_json
      current_path = "/api/v1/collections/#{id}/add"
      @conn.post(current_path, payload)
    end
adds site(s) to a site collection
 @param [int] id of the collection to which to add sites
 @param [int|Array] sites nid or array of site nids.
 @return [Hash{ "id" => Integer,
                "name" => String,
                "time" => "2016-10-28T09:25:26+00:00",
                "site_ids_added" => Array,
                "added" => Boolean,
                "message" => String }]