Add some more fluent APIs

[richardszalay]

Currently, creating a mock HttpClient looks like this:

var mockHandler = new MockHttpMessageHandler();
mockHandler.When("urAl").Respond(responseA);
mockHandler.When("urlA").Respond(responseB);
var httpClient = new HttpClient(mockHandler);

This could be made cleaner, at least for single-url mocks, via an AsHttpClient() method:

var httpClient = new MockHttpMessageHandler()
    .When("urlA").Respond(responseA)
    .AsHttpClient();

This could be taken one step further by having Respond return the MockHttpMessageHandler, allowing the following:

var httpClient = new MockHttpMessageHandler()
    .When("urlA").Respond(responseA)
    .When("urlB").Respond(responseB)
    .AsHttpClient();

It would even work for complex requirements with some imaginative spacing:

var httpClient = new MockHttpMessageHandler()
    .When("urlA")
        .WithQueryString("key", "value")
        .Respond(responseA)
    .When("urlB")
        .Respond(responseB)
    .AsHttpClient();

It's important that these changes be made in a backward-compatible fashion.