Test against Express 4.0; don't overwrite previous Set-Cookie header values

First, this change moves from Connect to Express for testing. Because if this project is meant to replace previously-bundled middleware for those who have moved to 4.x, it should be tested against 4.x. There are behavior differences now between the two because connect patches core http methods.

One of those behavior differences is `res.setHeader()`. `res.setHeader()`, when connect has been loaded, properly checks the previous value and creates an array or adds to the existing array. Under raw express 4.x, this module's cookie-setting logic overwrites previous sets of the Set-Cookie header, because it runs at the end of the request (by wrapping writeHead), and relies on that connect-provided protection. 

This change follows express's `setCookie()` solution for multiple cookies set in one middleware chain: https://github.com/visionmedia/express/blob/master/lib/response.js#L599