Fix CreateGuildAsync not sending icon stream. (#768)

* 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)

* Merged with old method

* Removed duplicate decl

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);
         }