public async Task FlushAsyncDoesNotThrow()
 {
     var stream = new FrameRequestStream();
     await stream.FlushAsync();
 }
 public async Task FlushAsyncThrows()
 {
     var stream = new FrameRequestStream(Mock.Of <IHttpBodyControlFeature>());
     await Assert.ThrowsAsync <NotSupportedException>(() => stream.FlushAsync());
 }