Add support for HAProxy ProxyProtocol

[aslafy-z]

Proposal

Add support for making requests with HAProxy proxy-protocol enabled.

Background

In some conditions, you may want to contact a reverse proxy that only accepts proxy protocol wrapped requests.
In my case, i'm trying to understand performances issues for my proxy-protocol enabled nginx ingress controller.

See sample client impl at https://github.com/mastercactapus/proxyprotocol/blob/master/cmd/proxy-get/main.go#L58

Workarounds

N/A