public IEnumerable<IConvertionRate> ListConvertionRates(DateTime? date = null)
        {
            if (date.HasValue)
            {
                throw new NotSupportedException("only allowed null value");
            }
            var currencyPairs = GetCurrencyPairs().ToArray();
            var convertionRates = new List<IConvertionRate>();
            Parallel.For(0, (int) Math.Ceiling((double) currencyPairs.Length/PartSize), i =>
            {
                var client = new RestClient("https://query.yahooapis.com/v1/public");
                var request = new RestRequest(
                    string.Format(RequestFormatString, GetQuery(currencyPairs.Skip(PartSize*i).Take(PartSize))));
                var response = client.Get(request);
                var rates = JObject.Parse(response.Content)
                    .SelectToken("query.results.rate")
                    .Select(GetConvertionRate);
                lock (_lock)
                {
                    convertionRates.AddRange(rates);
                }
            });
            return convertionRates;
        }