writeHead doesn't work as expected when passing headers as second parameter

Hey,

first of all: thanks for providing this library!

I ran into an issue with another library where writeHead is called like this:

res.writeHead(302, {
   'Access-Control-Allow-Origin': '*',
   'Location': path
});

It took me some time to realize, that azure-function-express provides its own implementation of this method. It seems that there is a incompatibility, as the original one allows for the second statusMessage to be omitted as you can see in the Node documentation:

const body = 'hello world';
response.writeHead(200, {
  'Content-Length': Buffer.byteLength(body),
  'Content-Type': 'text/plain' });

https://nodejs.org/api/http.html#http_response_writehead_statuscode_statusmessage_headers

The implementation provided by azure-function-express doesn’t support this call signature:

function writeHead(context, statusCode, statusMessage, headers) {
  // 1. Status code
  statusCode |= 0; // eslint-disable-line no-param-reassign
  if (statusCode < 100 || statusCode > 999) {
    throw new RangeError(`Invalid status code: ${statusCode}`);
  }

  // 2. Status message
 this.statusMessage = statusMessage || statusCodes[statusCode] || "unknown";

  // 3. Headers
  if (this._headers) {
    // Slow-case: when progressive API and header fields are passed.
    if (headers) {
      const keys = Object.keys(headers);
      for (let i = 0; i < keys.length; i++) {
        const k = keys[i];
        if (k) {
          this.setHeader(k, headers[k]);
        }
      }
    }
    // only progressive api is used
    headers = this._renderHeaders(); // eslint-disable-line no-param-reassign
  }

  // 4. Sets everything
  context.res.status = statusCode;
  context.res.headers = headers;
}

https://github.com/yvele/azure-function-express/blob/master/src/OutgoingMessage.js#L44

[gunzip]

@yvele you were right, it depends on node version (it works against 6.x)

[yvele]

@gunzip what version of Node.js are you using? Because it’s only compatible with 6.x.x