Use cancellationToken in two places it was missed. (#1539)

I'm not sure if this was a simple slip up, or if I was thinking of
skipping the check for speed reasons when I did this, but as these
two calls can both often happen at as the first actual write to stream,
which can have the longest delay waiting for the underlying layer,
they should both have the opportunity to cancel.

Src/Newtonsoft.Json/JsonTextWriter.Async.cs

@@ -407,7 +407,7 @@ namespace Newtonsoft.Json
         {
             await task.ConfigureAwait(false);
 
-            await _writer.WriteAsync('[').ConfigureAwait(false);
+            await _writer.WriteAsync('[', cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>
@@ -437,7 +437,7 @@ namespace Newtonsoft.Json
         {
             await task.ConfigureAwait(false);
 
-            await _writer.WriteAsync('{').ConfigureAwait(false);
+            await _writer.WriteAsync('{', cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>