Add ability to use StartsWith

[xxyuze]

public static class CloudExtensions
    {

        public static async Task<IEnumerable<TElement>> StartsWith<TElement>
        (this CloudTable table, string partitionKey, string searchStr,
        string columnName = "RowKey") where TElement : ITableEntity, new()
        {
            if (string.IsNullOrEmpty(searchStr)) return null;

            char lastChar = searchStr[searchStr.Length - 1];
            char nextLastChar = (char)((int)lastChar + 1);
            string nextSearchStr = searchStr.Substring(0, searchStr.Length - 1) + nextLastChar;
            string prefixCondition = TableQuery.CombineFilters(
                TableQuery.GenerateFilterCondition(columnName, QueryComparisons.GreaterThanOrEqual, searchStr),
                TableOperators.And,
                TableQuery.GenerateFilterCondition(columnName, QueryComparisons.LessThan, nextSearchStr)
                );

            string filterString = TableQuery.CombineFilters(
                TableQuery.GenerateFilterCondition("PartitionKey", QueryComparisons.Equal, partitionKey),
                TableOperators.And,
                prefixCondition
                );
            var query = new TableQuery<TElement>().Where(filterString);
            return await table.ExecuteQuerySegmentedAsync<TElement>(query,null);
        }
    }