"Modify Response" example code has problems

Created by: nilligan

This section of the README: https://github.com/nodejitsu/node-http-proxy#modify-response

It suggests that you should use body = Buffer.concat([body, data]); for every chunk received. This causes performance issues for large requests, since the Buffer.concat unnecessarily creates a new copy of the Buffer on every chunk.

The NodeJS docs suggest a different approach: pushing each chunk to a mutable array, with only one Buffer.concat in the "end" event: https://nodejs.org/en/docs/guides/anatomy-of-an-http-transaction/#request-body

let body = [];
request.on('data', (chunk) => {
  body.push(chunk);
}).on('end', () => {
  body = Buffer.concat(body).toString();
});

Happy to submit a PR