Merge pull request #6416 from jtattermusch/csharp_api_polishing

src/csharp/Grpc.Core.Tests/ClientServerTest.cs

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.csproj

src/csharp/Grpc.Core.Tests/Internal/AsyncCallServerTest.cs

src/csharp/Grpc.Core.Tests/Internal/AsyncCallTest.cs

src/csharp/Grpc.Core.Tests/Internal/FakeNativeCall.cs

src/csharp/Grpc.Core/Internal/AsyncCall.cs

src/csharp/Grpc.Core/Internal/AsyncCallBase.cs

src/csharp/Grpc.Core/Internal/AsyncCallServer.cs

src/csharp/Grpc.Core/Internal/ClientResponseStream.cs

src/csharp/Grpc.Core/Internal/ServerCallHandler.cs

src/csharp/Grpc.Core/Internal/ServerRequestStream.cs

src/csharp/Grpc.Core/Internal/ServerResponseStream.cs

src/csharp/tests.json