CSHARP-4401: Test full support for enums in filters.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToFilterTranslators/ToFilterFieldTranslators/ConvertExpressionToFilterFieldTranslator.cs

@@ -35,14 +35,33 @@ public static AstFilterField Translate(TranslationContext context, UnaryExpressi
                 var fieldType = fieldSerializer.ValueType;
                 var targetType = expression.Type;
 
-                if (fieldType.IsEnum)
+                if (fieldType.IsEnumOrNullableEnum(out _, out var underlyingType))
                 {
-                    var enumType = fieldType;
-                    var enumUnderlyingType = enumType.GetEnumUnderlyingType();
-                    if (targetType == enumUnderlyingType)
+                    if (targetType.IsSameAsOrNullableOf(underlyingType))
                     {
-                        var enumUnderlyingTypeSerializer = EnumUnderlyingTypeSerializer.Create(fieldSerializer);
-                        return AstFilter.Field(field.Path, enumUnderlyingTypeSerializer);
+                        IBsonSerializer enumSerializer;
+                        if (fieldType.IsNullable())
+                        {
+                            var nullableSerializer = (INullableSerializer)fieldSerializer;
+                            enumSerializer = nullableSerializer.ValueSerializer;
+                        }
+                        else
+                        {
+                            enumSerializer = fieldSerializer;
+                        }
+
+                        IBsonSerializer targetSerializer;
+                        var enumUnderlyingTypeSerializer = EnumUnderlyingTypeSerializer.Create(enumSerializer);
+                        if (targetType.IsNullable())
+                        {
+                            targetSerializer = NullableSerializer.Create(enumUnderlyingTypeSerializer);
+                        }
+                        else
+                        {
+                            targetSerializer = enumUnderlyingTypeSerializer;
+                        }
+
+                        return AstFilter.Field(field.Path, targetSerializer);
                     }
                 }