undocumented breaking change in url.format

fixed.js

var url = require('url')
url.format({host: 'google.com', pathname: 'foo', slashes: true})
// > '//google.com/foo'

node-6.js

var url = require('url')
url.format({host: 'google.com', pathname: 'foo'})
// > '//google.com/foo'

node-8.js

var url = require('url')
url.format({host: 'google.com', pathname: 'foo'})
// > 'google.comfoo'

The issue nodejs/node#11103