[plt-scheme] Teeny little net/url bug

(url->string (string->url "?")) => ""

I can't seem to make a url structure where url->string produces the
valid URL "?" (meaning the same spot, with no query arguments). "" by
itself means the same spot with the same query arguments.

I would recommend adding something in the url structure contract that
allows the query portion of make-url be either (listof (cons/c symbol?
(or/c false/c string?))) or 'clear to produce a ? without any name/value
pairs.