Always attempt to set the Connection: close response header

[zanieb]

While working on #428, I pondered the idea that closing the connection should cause client-side complaints of broken connections. It looks like Task.close_on_finish is sometimes set to true alongside a Connection: close header and other times not. If headers have not been written yet, it makes sense to always include Connection: close when close_on_finish is set. This pull request adds a utility that sets both of these values together. This results in better connection handling for clients.

There are a couple caveats:

• If Connection already has a directive in the response headers, we will not set it
• If headers have already been sent to the client, there is of course nothing we can do

There is also not particularly simple way to ensure that people call this method correctly unless we want to introduce a property for this variable — but I'm not really into properties with side-effects.

I believe this change does not invalidate #428 but it does also resolve #427.