bugfix: use upstream proxy

[BernhardRode]

Handle toProxy option properly and get proxy settings from environment.

There is an option toProxy (true/false) for http-proxy. But it is never used afaiks.
This fix checks the option and uses the http/https proxy set in the environment variables.

Usage:

{
  "/api": {
    "target": "http://upstreamserver.net",
    "toProxy": true
  }
}

Sorry, for I had no time for unit tests (just needed a local hotfix ;) )... If you like the solution I could into this PR in more detail during the weekend.

[assiomatica]

Hi,

I have tested your patch, as is it's not working, below the correct code:

`if (options.toProxy) {
var upstreamProxyServer;
if (typeof options.toProxy === 'boolean') {
upstreamProxyServer = url.parse(
isSSL.test(options[forward || 'target'].protocol) ? process.env['https_proxy'] || '' : process.env['http_proxy'] || ''
);
} else {
upstreamProxyServer = url.parse(options.toProxy);
}

if (upstreamProxyServer) {
  outgoing.path = options[forward || 'target'].protocol + '//' + outgoing.host + outgoing.path;
  outgoing.host = upstreamProxyServer.host;
  outgoing.hostname = upstreamProxyServer.hostname;
  outgoing.port = upstreamProxyServer.port;
}

}
`