public async Task Login_sends_a_request_with_the_specified_credentials()
{
_requestSender
.SendAsync(Arg.Any <HttpRequestMessageTemplate>())
.Returns(ci => Response.GetToken(ci.Arg <HttpRequestMessageTemplate>()));
await _sut.FetchAccessTokenAsync();
await _requestSender
.Received(1)
.SendAsync(Arg.Is <HttpRequestMessageTemplate>(m =>
m.Method == HttpMethod.Post &&
IsFormWithClientCredentials(m.Content, ExpectedClientId, ExpectedClientSecret)));
}