Connection header fix for WebSocket

I have supressed the sending of `Connection: keep-alive` and `Connection: close` header values for the specific case of response status `101 Switching Protocols` to send `Connection: upgrade` instead. This is likely to better conform to the HTTP/1.1 and WebSocket specifications.

I did this to (in the near future) remove the weird workaround in current WebSocket middleware, which accesses the underlying connection to send the corrected header values.

![](https://travis-ci.org/ricardo-kagawa/meinheld.svg?branch=upgrade-connection)