https-proxy-agent: WIP buffer the incoming proxy data

In case we need to re-play the data onto the socket in the case of a
proxy error (so that the user can handle it).

This doesn't *fully* work yet, since it seems like in the 407 Authenticate
scenario, the socket never closes. Need to look into that some more...
package: depend on "proxy" >= v0.2.1

Fixes a bug in the "proxy" server code that was making the tests timeout
https-proxy-agent: remove console.error() call
test: re-enable the 407 response code test

It should pass now that the "proxy" server module is fixed.