Extend req/res mock to include send()
Some of the passport strategies out there use `req.res.send()` to achieve a redirect via automatic post (for example: [passport-saml](https://github.com/bergie/passport-saml/blob/2de3528f308f2103625b191c5b32432636f1592e/lib/passport-saml/strategy.js#L94)).

In order to support this, `koa-passport` should mock out the `send()` function as well as make the `res` mock available on the `req` mock.