bug#60528: uri->string swallows mandatory slashes

Hi folks,

Below some unexpected behaviour observed in Guile 3.0.8, but it also
dates back to 2.2.7 and possibly earlier versions.

scheme@(guile-user)> ,use (web uri)
scheme@(guile-user)> (string->uri "file:///home") 
$1 = #<<uri> scheme: file userinfo: #f host: #f port: #f path: "/home"
query: #f fragment: #f>
scheme@(guile-user)> (uri->string $1)
$2 = "file:/home"
scheme@(guile-user)> (uri->string (build-uri 'file #:host "" #:path
"/home")) 
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Throw to key `uri-error' with args `("Expected valid host: ~s" (""))'.

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.

Cheers