Added missing class fixture

Author: alexeyzimarev

test/RestSharp.IntegrationTests/AsyncRequestBodyTests.cs

@@ -4,7 +4,7 @@
 
 namespace RestSharp.IntegrationTests;
 
-public class AsyncRequestBodyTests {
+public class AsyncRequestBodyTests : IClassFixture<RequestBodyFixture> {
     public AsyncRequestBodyTests(RequestBodyFixture fixture) {
         var server = fixture.Server;
         _client = new RestClient(server.Url);

test/RestSharp.Serializers.Tests/NewtonsoftJsonTests.cs

@@ -11,7 +11,7 @@ namespace RestSharp.Serializers.Tests;
 public class NewtonsoftJsonTests {
     static readonly Fixture Fixture = new();
 
-    string? _body;
+    string _body;
 
     readonly JsonSerializerSettings _jsonSerializerSettings = new() {
         ContractResolver = new DefaultContractResolver {
@@ -48,7 +48,7 @@ public void Serialize_within_multiple_threads() {
         Parallel.For(
             0,
             100,
-            n => {
+            _ => {
                 var dummy                 = Fixture.Create<TestClass>();
                 var expectedSerialization = JsonConvert.SerializeObject(dummy, _jsonSerializerSettings);
                 var actualSerialization   = serializer.Serialize(dummy);