Include port in default Host header

See [RFC 2616 section 14.23](http://tools.ietf.org/html/rfc2616#section-14.23): if the port is not the default it should be included in the Host header.

Currently it is not and it actually results in `404` errors with strict HTTP servers like [the Flask stack with SERVER_NAME set](http://flask.pocoo.org/docs/config/).

This pull request fixes this by appending the port if needed.