more hop-by-hop fixes

This accompanies https://github.com/mitsuhiko/werkzeug/pull/96 to make it easier to comply with http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.10

My concern about this patch is it unconditionally removes headers matching the connection tokens without first checking what they are, making us vulnerable to bogus Connection headers. But I didn't see a set of headers which are unsafe to remove which we could check against. I could submit a better patch if you provide feedback.