There are only two cases in which I can see a developer wanting to specify `Connection`:

* they are using HTTP/1.0 and want to advertise that they support keep-alive
* they are using HTTP/1.1 but want to manually close the connection immediately after completing the request

In general, most developers will default to HTTP/1.1, and will likely want keep-alive, which is the assumed default for HTTP/1.1 (at least within `spray-can`).

We can let this one through, but it might be more prudent to treat keep-alive as a boolean somewhere instead.