Merge pull request #6794 from jtattermusch/csharp_misc_fixes

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

src/csharp/Grpc.Core/AsyncDuplexStreamingCall.cs

src/csharp/Grpc.Core/AsyncServerStreamingCall.cs

src/csharp/Grpc.Core/AsyncUnaryCall.cs

src/csharp/Grpc.Core/CallOptions.cs

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/ChannelState.cs

src/csharp/Grpc.Core/Server.cs

src/csharp/Grpc.Examples/MathGrpc.cs

src/proto/math/math.proto