Allow to use custom http modules(e.g unit-http)

[alherd-by]

This change will allow to use application server Nginx Unit(https://unit.nginx.org/installation/#installation-nodejs-package) in koa applications without any changes to existing codebases.

[codecov]

Codecov Report

Merging #1327 into master will decrease coverage by 0.5%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #1327      +/-   ##
==========================================
- Coverage     100%   99.49%   -0.51%     
==========================================
  Files           5        5              
  Lines         393      397       +4     
==========================================
+ Hits          393      395       +2     
- Misses          0        2       +2

Impacted Files	Coverage Δ
lib/application.js	98.01% <60%> (-1.99%)	⬇️

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[kroleg]

you can write httpModule = args.shift();

[kroleg]

i don't think that this should be included in koa because there is already an easy way to use custom http server:

const app = new Koa();
const unitHttp = require('unit-http');
const server = unitHttp.createServer(app.callback());
server.listen(port, () => {
    console.log('Listening on port ' + port);
});

[fl0w]

Aye, this is a no go from me. Use app.callback() // e.g. spdy.listen(app.callback())

[alherd-by]

Thank you for responses.