Use modern HttpClient

(cherry picked from commit 402f8b296f17bf161824ec5ff40d67d036d00d94)

src/NzbDrone.Common/Http/HttpHeader.cs

@@ -4,11 +4,27 @@ using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Globalization;
 using System.Linq;
+using System.Net;
+using System.Net.Http.Headers;
 using System.Text;
 using NzbDrone.Common.Extensions;
 
 namespace NzbDrone.Common.Http
 {
+    public static class WebHeaderCollectionExtensions
+    {
+        public static NameValueCollection ToNameValueCollection(this HttpHeaders headers)
+        {
+            var result = new NameValueCollection();
+            foreach (var header in headers)
+            {
+                result.Add(header.Key, header.Value.ConcatToString(";"));
+            }
+
+            return result;
+        }
+    }
+
     public class HttpHeader : NameValueCollection, IEnumerable<KeyValuePair<string, string>>, IEnumerable
     {
         public HttpHeader(NameValueCollection headers)
@@ -16,6 +32,11 @@ namespace NzbDrone.Common.Http
         {
         }
 
+        public HttpHeader(HttpHeaders headers)
+            : base(headers.ToNameValueCollection())
+        {
+        }
+
         public HttpHeader()
         {
         }