hop-by-hop fixes

`_hop_by_pop_headers` includes "trailers", but it should be "trailer" according to http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.40 and http://en.wikipedia.org/wiki/List_of_HTTP_header_fields.

Also, why "hop_by_pop" and not "hop_by_hop"?

Finally, http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.10 says: "HTTP/1.1 proxies MUST parse the Connection header field before a message is forwarded and, for each connection-token in this field, remove any header field(s) from the message with the same name as the connection-token." Werkzeug is not currently doing this. Shouldn't it?