eShopOnWeb PR to allow basket items removal

I did a PR request on forked Microsoft's demo project eShopOnWeb to allow basket items removal by setting the quantity to zero. See the commit 70e009b.

Also, added 2 more tests (a unit and another integration test) for new functionality. Plus another small fix in some unit testing async functions to return Task instead of void.

This sample project is a good example of architectural principles, very well described in the eBook that comes with the project. I'm thinking to do some other contributions to this project, as coding exercises.