Use KeyValuePairSerializer.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/KnownSerializerFinders/KnownSerializerFinderVisitMethodCall.cs

@@ -1637,12 +1637,7 @@ void DeduceCreateMethodSerializers()
                 {
                     var keySerializer = argumentSerializers[0];
                     var valueSerializer = argumentSerializers[1];
-                    var keyValuePairSerializer = NewKeyValuePairExpressionToAggregationExpressionTranslator.CreateResultSerializer(
-                        resultType: method.ReturnType,
-                        keySerializer,
-                        valueSerializer,
-                        out _,
-                        out _);
+                    var keyValuePairSerializer = KeyValuePairSerializer.Create(BsonType.Document, keySerializer, valueSerializer);
                     AddKnownSerializer(node, keyValuePairSerializer);
                 }
             }

src/MongoDB.Driver/Linq/Linq3Implementation/KnownSerializerFinders/KnownSerializerFinderVisitNew.cs

@@ -118,12 +118,7 @@ IBsonSerializer GetKnownSerializer(ConstructorInfo constructor)
                 if (IsKnown(key, out var keySerializer) &&
                     IsKnown(value, out var valueSerializer))
                 {
-                    return NewKeyValuePairExpressionToAggregationExpressionTranslator.CreateResultSerializer(
-                        resultType: node.Type,
-                        keySerializer,
-                        valueSerializer,
-                        out _,
-                        out _);
+                    return KeyValuePairSerializer.Create(BsonType.Document, keySerializer, valueSerializer);
                 }
             }
             else if (TupleOrValueTupleConstructor.IsTupleOrValueTupleConstructor(constructor))

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/NewKeyValuePairExpressionToAggregationExpressionTranslator.cs

@@ -13,13 +13,12 @@
 * limitations under the License.
 */
 
-using System;
 using System.Collections.Generic;
 using System.Linq.Expressions;
 using MongoDB.Bson;
-using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Serializers;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
+using MongoDB.Driver.Linq.Linq3Implementation.Serializers;
 
 namespace MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators
 {