Fix up ASP.NET formatter to run on .NET Core 3.1

src/MessagePack.AspNetCoreMvcFormatter/MessagePackOutputFormatter.cs

@@ -34,6 +34,12 @@ namespace MessagePack.AspNetCoreMvcFormatter
                     return Task.CompletedTask;
 #else
                     var writer = context.HttpContext.Response.BodyWriter;
+                    if (writer == null)
+                    {
+                        context.HttpContext.Response.Body.WriteByte(MessagePackCode.Nil);
+                        return Task.CompletedTask;
+                    }
+
                     var span = writer.GetSpan(1);
                     span[0] = MessagePackCode.Nil;
                     writer.Advance(1);
@@ -46,6 +52,11 @@ namespace MessagePack.AspNetCoreMvcFormatter
                     return MessagePackSerializer.SerializeAsync(context.Object.GetType(), context.HttpContext.Response.Body, context.Object, this.options, context.HttpContext.RequestAborted);
 #else
                     var writer = context.HttpContext.Response.BodyWriter;
+                    if (writer == null)
+                    {
+                        return MessagePackSerializer.SerializeAsync(context.Object.GetType(), context.HttpContext.Response.Body, context.Object, this.options, context.HttpContext.RequestAborted);
+                    }
+
                     MessagePackSerializer.Serialize(context.Object.GetType(), writer, context.Object, this.options, context.HttpContext.RequestAborted);
                     return writer.FlushAsync().AsTask();
 #endif
@@ -57,6 +68,11 @@ namespace MessagePack.AspNetCoreMvcFormatter
                 return MessagePackSerializer.SerializeAsync(context.ObjectType, context.HttpContext.Response.Body, context.Object, this.options, context.HttpContext.RequestAborted);
 #else
                 var writer = context.HttpContext.Response.BodyWriter;
+                if (writer == null)
+                {
+                    return MessagePackSerializer.SerializeAsync(context.ObjectType, context.HttpContext.Response.Body, context.Object, this.options, context.HttpContext.RequestAborted);
+                }
+
                 MessagePackSerializer.Serialize(context.ObjectType, writer, context.Object, this.options, context.HttpContext.RequestAborted);
                 return writer.FlushAsync().AsTask();
 #endif

tests/MessagePack.AspNetCoreMvcFormatter.Tests/MessagePack.AspNetCoreMvcFormatter.Tests.csproj

@@ -8,7 +8,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.Mvc.Core" Version="2.2.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.0" />
     <PackageReference Include="Moq" Version="4.10.1" />
   </ItemGroup>