ExCon doesn't handle Keep-Alive properly

[jdunck]

:persistent => false is the default, meaning TCP connection-per-request.

:persistent => true, unfortunately, does not look at the Keep-Alive header, it merely looks for Connection: close and if it doesn't find it, keeps the TCP connection forever. This means 1) for servers that don't support Keep-Alive at all, things are likely to break, 2) for servers that have a short Keep-Alive timeout but not an explicit Connection close, things break eventually.

If we want to benefit from reduced TCP overhead, the way to handle this nicely would be to fix ExCon. We could also work around it by catching the exception that comes out of ExCon and re-trying with a new connection.