Handle automatic reconnects of `HTTP::Client`

[syeopite]

When a long running connection is closed by the server HTTP::Client will try to reconnect by closing its @io and then constructing a new @io via its private io method which will reset the client back to the socket the standard HTTP::Client uses.

This means that a long running client that is using an HTTP proxy could become unproxied after some time.

Could this behavior be handled somehow?