CSHARP-4429: Support resultSerializer = null in MongoQueryable.AppendStage (mongodb#972)

Author: BorisDog

src/MongoDB.Driver/Linq/MongoQueryable.cs

@@ -87,7 +87,7 @@ public static IMongoQueryable<TResult> AppendStage<TSource, TResult>(
                     GetMethodInfo(AppendStage, source, stage, resultSerializer),
                     Expression.Convert(source.Expression, typeof(IMongoQueryable<TSource>)),
                     Expression.Constant(stage),
-                    Expression.Constant(resultSerializer)));
+                    Expression.Constant(resultSerializer, typeof(IBsonSerializer<TSource>))));
         }
 
         /// <summary>

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4234Tests.cs

@@ -16,19 +16,21 @@
 using System.Linq;
 using FluentAssertions;
 using MongoDB.Bson;
+using MongoDB.Bson.TestHelpers.XunitExtensions;
 using MongoDB.Driver.Linq;
 using Xunit;
 
 namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
 {
     public class CSharp4234Tests : Linq3IntegrationTest
     {
-        [Fact]
-        public void AppendStage_should_work()
+        [Theory]
+        [ParameterAttributeData]
+        public void AppendStage_should_work([Values(false, true)] bool useResultSerializer)
         {
             var collection = CreateProductsCollection();
             var textStage = "{ $match : { $text : { $search : 'apples' } } }";
-            var resultSerializer = collection.DocumentSerializer;
+            var resultSerializer = useResultSerializer ? collection.DocumentSerializer : null;
 
             var queryable = collection
                 .AsQueryable()