public async Task Given_Post_InvalidParameter_When_TryToMakeAPurchase_Then_BadRequestIsReturned()
{
// arrange
// act
var actionResult = await purchaseController.PostPurchase(null);
// assert
Assert.True(actionResult.Result is BadRequestObjectResult);
Assert.Equal((actionResult.Result as BadRequestObjectResult).StatusCode, (int)HttpStatusCode.BadRequest);
}