better URI syntax complience

[dstrop]

Nette\Http\Url::getHostUrl() returns '//' when no scheme or host is suplied.
Which is slightly annoing for me and it goes against URI syntax 'scheme:[//[user:password@]host[:port]][/]path[?query][#fragment]'.

[Majkl578]

Leading // is mandatory if authority is present in the URI (regardless if scheme is present or not), as per RFC 3986.

Also PSR-7 follows this:

If an authority is present, it MUST be prefixed by "//".

[dg]

For http: it should be omitted, for file: is should not https://en.wikipedia.org/wiki/File_URI_scheme.