Small fixes

Author: papafe

src/MongoDB.Driver/Authentication/MongoDBX509Authenticator.cs

@@ -113,7 +113,7 @@ private CommandWireProtocol<BsonDocument> CreateAuthenticateProtocol()
                 resultSerializer: BsonDocumentSerializer.Instance,
                 messageEncoderSettings: null,
                 serverApi: _serverApi,
-                serializationDomain: BsonSerializer.DefaultSerializationDomain); //QUESTION Is this correct? Using a default serialization domain?
+                serializationDomain: BsonSerializer.DefaultSerializationDomain);
 
             return protocol;
         }

src/MongoDB.Driver/Core/WireProtocol/Messages/Encoders/BinaryEncoders/ClientBulkWriteOpsSectionFormatter.cs

@@ -29,9 +29,9 @@ internal sealed class ClientBulkWriteOpsSectionFormatter : ICommandMessageSectio
     {
         private readonly long? _maxSize;
         private readonly Dictionary<string, int> _nsInfos;
+        private readonly IBsonSerializationDomain _serializationDomain;
         private MemoryStream _nsInfoMemoryStream;
         private BsonBinaryWriter _nsInfoWriter;
-        private IBsonSerializationDomain _serializationDomain;
         private Dictionary<int, object> _idsMap;
         private int _currentIndex;
 

src/MongoDB.Driver/Core/WireProtocol/Messages/Encoders/BinaryEncoders/QueryMessageBinaryEncoder.cs

@@ -156,7 +156,6 @@ private void WriteOptionalFields(BsonBinaryWriter binaryWriter, BsonDocument fie
         {
             if (fields != null)
             {
-                //QUESTION Is it correct we only need a default domain here?
                 var context = BsonSerializationContext.CreateRoot(binaryWriter, BsonSerializer.DefaultSerializationDomain);
                 BsonDocumentSerializer.Instance.Serialize(context, fields);
             }