Fix first sample in Readme.md

Line 26 was `res.send(200, {...})` but would give this warning when running:
`express deprecated res.send(status, body): Use res.status(status).send(body) instead`

Line 32 was `.expect('Content-Length', '15')` but it gave me an error:
`Error: expected "Content-Length" matching 20, got "15"`

I've fixed those.

(using Express 4.10.3)