flushHeaders is not implemented on response mock

[RobinTail]

TypeError: response.type(...).setHeader(...).setHeader(...).flushHeaders is not a function

It should just set headersSent on response, as per my understanding

[RobinTail]

As a workaround writeHead() method can be used instead of type() +setHeader() + flushHeaders() ones.

[RobinTail]

But writeHead does not set headersSent prop to true in the mocked implementation.

node-mocks-http/lib/mockResponse.js

Line 108 in 7297984

mockResponse.writeHead = function writeHead(statusCode, statusMessage, headers) {

I think it should https://nodejs.org/api/http.html#responsewriteheadstatuscode-statusmessage-headers

[RobinTail]

I checked manually to ensure the actual behaviour of writeHead() and it really does set headersSent to true, because it sends the headers. So I made a PR to fix that.