"cant set headers after they are sent" occured in 2.23.2 but not in 2.21.0

[jiadi0801]

I use browser-sync as the server in my project since v2.18.13. Yesterday, I reinstalled my project. When I run it, the cli print:

_http_outgoing.js:494
    throw new Error('Can\'t set headers after they are sent.');

env

• Browsersync [ 2.23.2 and 2.21.0 ]
• Node [ 8.9.3 ]
• windows 7
• Git Bash

test case

// const browserSync = require('../bs/node_modules/browser-sync/');   // for bs 2.23.2
const browserSync = require('../bs21/node_modules/browser-sync/');  // for bs 2.21.0

let bs = browserSync.create('bsServer');
bs.init({
	server: '',
    open: true,
    middleware: [mw]
});
function mw(request, response, next) {
    response.write('bs');
    response.end();
    next(); 
}

when use v2.23.2, the server will stop, and throw the error below.
v2.21.0 works fine.

_http_outgoing.js:494
    throw new Error('Can\'t set headers after they are sent.');
    ^

Error: Can't set headers after they are sent.
    at validateHeader (_http_outgoing.js:494:11)
    at ServerResponse.setHeader (_http_outgoing.js:501:3)
    at D:\TMP\bs\node_modules\serve-index\index.js:213:13
    at D:\TMP\bs\node_modules\serve-index\index.js:324:7
    at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:511:3)

I cannot call response.end or response.write method in my middleware.
If I have to call these methods, I have to remove the next() method. This seems to prevent the middleware flow.

I want to know how this problem arises, thanks!

[shakyShane]

I've pushed a fix that will work for you when doing the example above

Note: It will still not work if you do something like server: 'app' as this will append a server-static middleware to the end of the stack and will result in the same bug