Face detection and uploading source file to Rekognition service from C# .NET Async

Program.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace HelloWorldRekognition
{
    class Program
    {
        private const string ApiKey = "";
        private const string ApiSecretKey = "";

        static void Main()
        {
            Call().ContinueWith(task => Console.WriteLine(task.Result));
            
            Console.ReadLine();
        }

        private static async Task<string> Call()
        {
            var data = new Dictionary<string, string>
            {
                {"api_key", ApiKey},
                {"api_secret", ApiSecretKey},
                {"jobs", "face_gender_age_emotion_recognize"},
                {"base64", Convert.ToBase64String(File.ReadAllBytes("people.jpg"))}
            };

            var content = new BodyFormUrlEncodedContent(data);

            var request = new HttpRequestMessage
            {
                Content = content,
                Method = HttpMethod.Post,
                RequestUri = new Uri("https://rekognition.com/func/api/")
            };

            var httpClient = new HttpClient();
            var sendTask = await httpClient.SendAsync(request);
            var response = await sendTask.Content.ReadAsStringAsync();
            return response;
        }
    }

    public class BodyFormUrlEncodedContent : ByteArrayContent
    {
        public BodyFormUrlEncodedContent(IEnumerable<KeyValuePair<string, string>> nameValueCollection)
            : base(GetContentByteArray(nameValueCollection))
        {
            Headers.ContentType = new MediaTypeHeaderValue("application/x-www-form-urlencoded");
        }

        private static byte[] GetContentByteArray(IEnumerable<KeyValuePair<string, string>> nameValueCollection)
        {
            if (nameValueCollection == null)
            {
                throw new ArgumentNullException("nameValueCollection");
            }

            StringBuilder stringBuilder = new StringBuilder();
            foreach (KeyValuePair<string, string> current in nameValueCollection)
            {
                if (stringBuilder.Length > 0)
                {
                    stringBuilder.Append('&');
                }

                stringBuilder.Append(Encode(current.Key));
                stringBuilder.Append('=');
                stringBuilder.Append(Encode(current.Value));
            }
            return Encoding.Default.GetBytes(stringBuilder.ToString());
        }
        private static string Encode(string data)
        {
            if (string.IsNullOrEmpty(data))
            {
                return string.Empty;
            }
            return (WebUtility.UrlEncode(data) ?? string.Empty).Replace("%20", "+");
        }
    }
}