Use actual request type in new 'Send' method

[remcoros]

See this old issue for background: #124

var requests = new IRequest[] { new Req1(), new Req2(), new Req3() };
foreach (var req in requests)
{
  await _mediator.Send(req);
}

It will try to resolve handlers of type "IRequest" instead of using the concrete type of the given request.

sorry I didn't catch that yesterday 😞 let me know if I should add a test

[jbogard]

Can you add a test for this? So I don't break it again.

[jbogard]

I'll wait a bit to see if there are any other bugs before making a release for this.