make ->string work with URLs

sugar/test/main.rkt

@@ -42,7 +42,7 @@
  (check-equal? (->string (void)) "")
  (check-equal? (->string 'foo) "foo")
  (check-equal? (->string 123) "123")
- ;(check-equal? (->string (string->url "foo/bar.html")) "foo/bar.html")
+ (check-equal? (->string (string->url "foo/bar.html")) "foo/bar.html")
  (define file-name-as-text "foo.txt")
  (check-equal? (->string (string->path file-name-as-text)) file-name-as-text)
  (check-equal? (->string #\¶) "¶")

typed/sugar/coerce.rkt

@@ -23,7 +23,7 @@
 
 
 (provide Stringish)
-(define-type Stringish (U String Symbol Number Path Char Null Void))
+(define-type Stringish (U String Symbol Number Path Char Null Void SugarURL))
 
 
 (define/typed+provide (->string x)
@@ -37,6 +37,7 @@
           [(number? x) (number->string x)]
           [(path? x) (path->string x)]
           [(char? x) (format "~a" x)]
+          [(url? x) (url->string x)]
           [else (error 'bad-type)]))))