Proxy Headers get lost here.

[mlattanzio]

https://github.com/koichik/node-tunnel/blob/5fb2fb424788597146b7be6729006cad1cf9e9a8/lib/tunnel.js#L108

mergeOptions leaves the proxyOptions.headers behind and they don't get send to the proxy server.

An ugly example fix would be Object.assign(connectOptions.headers, self.proxyOptions.headers) on the following line