MockHttp for HttpClient

MockHttp is a testing layer for Microsoft's HttpClient library. It allows stubbed responses to be configured for matched HTTP requests and can be used to test your application's service layer. MockHttp defines a replacement HttpMessageHandler, the engine that drives HttpClient, that provides a fluent configuration API and provides a canned response. The caller (eg. your application's service layer) remains unaware of its presence. MockHttpMessageHandler defines both When and Expect, which can be used to define responses. They both expose the same fluent API, but each works in a slightly different way. Using When specifies a "Backend Definition". Backend Definitions can be matched against multiple times and in any order, but they won't match if there are any outstanding Request Expectations present (unless BackendDefinitionBehavior.Always is specified).