added ResponseHeadersTest, fixed stylecop issues

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

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

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

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

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

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

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

src/csharp/Grpc.Core/WriteOptions.cs