url: inconsistence with docs on slashed protocols

[claudiorodriguez]

In the docs, at https://nodejs.org/api/url.html#url_url_format_urlobj it says:

"protocol" is treated the same with or without the trailing : (colon).
The protocols http, https, ftp, gopher, file will be postfixed with :// (colon-slash-slash).
All other protocols mailto, xmpp, aim, sftp, foo, etc will be postfixed with : (colon).
"slashes" set to true if the protocol requires :// (colon-slash-slash)
Only needs to be set for protocols not previously listed as requiring slashes, such as mongodb://localhost:8000/.

This is not how the code works. If you look at https://github.com/nodejs/node/blob/master/lib/url.js you see:

  // only the slashedProtocols get the //.  Not mailto:, xmpp:, etc.
  // unless they had them to begin with.
  if (this.slashes ||
      (!protocol || slashedProtocol[protocol]) && host !== false) {
    host = '//' + (host || '');
    if (pathname && pathname.charAt(0) !== '/') pathname = '/' + pathname;
  } else if (!host) {
    host = '';
  }

So the documented behaviour only happens when host/hostname are set.
You can check this with the following snippet:

var url = require('url');

var unslashedUri = url.parse('/no-slashes');
var slashedUriByHost = url.parse('/slashes-host');
var slashedUriForced = url.parse('/slashes-forced');

unslashedUri.protocol = 'file';
console.log(url.format(unslashedUri)); // file:/no-slashes

slashedUriByHost.protocol = 'file';
slashedUriByHost.host = 'localhost';
console.log(url.format(slashedUriByHost)); // file://localhost/slashes-host

slashedUriForced.protocol = 'file';
slashedUriForced.slashes = true
console.log(url.format(slashedUriForced)); // file:///slashes-forced

I can make a PR correcting either the docs, or the url module, but which one is the intended behaviour?

[claudiorodriguez]

Duplicate of #3361 actually