public async Task Given_A_RemoteFileUrl_And_A_LocalFileFullPath_Should_Invoke_Download_Method_Once()
{
// Arrange
var remoteFileUrl = new Uri("http://www.google.com");
const string localFileFullPath = @"c:\images\pic.png";
// Act
await _sut.Download(remoteFileUrl, localFileFullPath);
// Assert
await _fileSystem.Received(1).Download(Arg.Is(remoteFileUrl), Arg.Is(localFileFullPath));
}