Add default http client

MediaBrowser.Common/Net/UserAgentDelegatingHandler.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Net.Http;
+using System.Net.Http.Headers;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MediaBrowser.Common.Net
+{
+    /// <summary>
+    /// User agent delegating handler.
+    /// Adds User-Agent header to all requests.
+    /// </summary>
+    public class UserAgentDelegatingHandler : DelegatingHandler
+    {
+        /// <inheritdoc />
+        public UserAgentDelegatingHandler(IApplicationHost applicationHost)
+        {
+            UserAgentValues = new List<ProductInfoHeaderValue>
+            {
+                new ProductInfoHeaderValue(applicationHost.Name.Replace(' ', '-'),  applicationHost.ApplicationVersionString),
+                new ProductInfoHeaderValue($"({Environment.OSVersion}; {applicationHost.ApplicationUserAgentAddress})")
+            };
+        }
+
+        /// <summary>
+        /// Gets or sets the user agent values.
+        /// </summary>
+        public List<ProductInfoHeaderValue> UserAgentValues { get; set; }
+
+        /// <summary>
+        /// Send request message.
+        /// </summary>
+        /// <param name="request">The request message.</param>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>A <see cref="Task"/> containing the <see cref="HttpResponseMessage"/>.</returns>
+        protected override Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request,
+            CancellationToken cancellationToken)
+        {
+            if (request.Headers.UserAgent.Count == 0)
+            {
+                foreach (var userAgentValue in UserAgentValues)
+                {
+                    request.Headers.UserAgent.Add(userAgentValue);
+                }
+            }
+
+            return base.SendAsync(request, cancellationToken);
+        }
+    }
+}