Support PROXY protocol for HTTP health checks

We are using Consul to expose a HTTP service that is backed by nginx. Nginx is listening on ports 80 and 443 using [`PROXY` protocol](https://docs.nginx.com/nginx/admin-guide/load-balancer/using-proxy-protocol/). Ideally we would configure a `http` health check for this service, but we can't do so because the `http` health check doesn't support `PROXY` protocol. Without `PROXY` protocol, the HTTP check fails:

```
Mar 01 21:53:21 ip-10-7-28-60 consul[6248]: 2018/03/01 21:53:21 [WARN] agent: http request failed 'http://127.0.0.1:80': Get http://127.0.0.1:80: EOF
```