Running 'setHeader' during 'proxyReq' event fails for POST requests

Created by: lrettig

Hello,

I'm running into the same issue that happened in #908 which was supposed to have been solved already, but it's occurring for me now on the latest v1.16.2. Trying to do a setHeader on the passed in proxyReq for a POST requests fails with:

Error: Can't set headers after they are sent.

It's happening for both node 4.3.2 and 6.1.0.

To reproduce:

proxy.on('proxyReq', function(proxyReq, req, res, options) {
  proxyReq.setHeader('origin', 'https://example.com');
});

Thank you.