Fix incorrect response object being emitted on redirect

[wilsonjackson]

Changes the argument emitted with the redirect event to a Supertest Response object instead of an unwrapped IncomingMessage.

Closes #1760

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[wilsonjackson]

@titanism This is the main change I'm not super certain about. Without doing this, a couple of the tests around .pipe() fail because there's no this.res on the response. I don't know if this assignment has the potential to screw anything else up.

[titanism]

v8.0.9 released to npm and GitHub releases

https://github.com/ladjs/superagent/releases/tag/v8.0.9