Fix CreateGuildAsync not doing anything with the input stream for the guild icon.

Also fixes an issue with potential stream types that throw a NotSupportedException when checking its properties. [Apparently, they exist.](https://github.com/dotnet/corefx/blob/master/src/System.Net.Http.WinHttpHandler/src/System/Net/Http/WinHttpResponseStream.cs)

src/Discord.Net.Rest/ClientHelper.cs

@@ -120,6 +120,9 @@ namespace Discord.Rest
             string name, IVoiceRegion region, Stream jpegIcon, RequestOptions options)
         {
             var args = new CreateGuildParams(name, region.Id);
+            if (jpegIcon != null)
+                args.Icon = new API.Image(jpegIcon);
+
             var model = await client.ApiClient.CreateGuildAsync(args, options).ConfigureAwait(false);
             return RestGuild.Create(client, model);
         }

src/Discord.Net.Rest/Net/Converters/ImageConverter.cs

@@ -1,5 +1,6 @@
-using Newtonsoft.Json;
-using System;
+using System;
+using System.IO;
+using Newtonsoft.Json;
 using Model = Discord.API.Image;
 
 namespace Discord.Net.Converters
@@ -23,8 +24,12 @@ namespace Discord.Net.Converters
 
             if (image.Stream != null)
             {
-                byte[] bytes = new byte[image.Stream.Length - image.Stream.Position];
-                image.Stream.Read(bytes, 0, bytes.Length);
+                Stream cloneStream = new MemoryStream();
+                image.Stream.CopyTo(cloneStream);
+
+                byte[] bytes = new byte[cloneStream.Length];
+                cloneStream.Seek(0, SeekOrigin.Begin);
+                cloneStream.Read(bytes, 0, bytes.Length);
 
                 string base64 = Convert.ToBase64String(bytes);
                 writer.WriteValue($"data:image/jpeg;base64,{base64}");