Feature request: expose collected requests

[johnknoop]

Hi and thanks for a great library!

In order to have good failure messages in my tests, I generally do this:

httpMock.Fallback
    .Respond(req => throw new Exception($"No matcher found: {req.Method} {req.RequestUri.PathAndQuery}"));

But some of my tests contain tens of different HTTP requests, and in order for a failing test to point me in the right direction, it would be great if I could get the last matched request from the mock, so that I can add that to my failure message, like:

Last successful HTTP request: POST /somepath?my=query