url: fix parsing of relative urls in string->url/literal

Also, explicitly check for relative Location urls.

http-easy-lib/http-easy/private/session.rkt

@@ -282,12 +282,15 @@
     (string->url/literal location))
   (cond
     [(url-host location-url) location-url]
-    [else (struct-copy
-           url/literal location-url
-           [scheme #:parent url (url-scheme orig)]
-           [user #:parent url (url-user orig)]
-           [host #:parent url (url-host orig)]
-           [port #:parent url (url-port orig)])]))
+    [(not (url-path-absolute? location-url))
+     (error 'ensure-absolute-url "Location destination is relative")]
+    [else
+     (struct-copy
+      url/literal location-url
+      [scheme #:parent url (url-scheme orig)]
+      [user #:parent url (url-user orig)]
+      [host #:parent url (url-host orig)]
+      [port #:parent url (url-port orig)])]))
 
 (define (same-origin? a b)
   (and

http-easy-lib/http-easy/private/url.rkt

@@ -55,7 +55,7 @@
     (url/literal scheme user host port abs? path query fragment)))
 
 (define (url/literal->string u)
-  (match-define (url scheme user host port _ path query fragment) u)
+  (match-define (url scheme user host port abs? path query fragment) u)
   (call-with-output-string
    (lambda (out)
      (when scheme
@@ -82,17 +82,19 @@
         (write-string "//" out)]
        [else
         (void)])
-     (let loop ([path-components path])
-       (unless (null? path-components)
-         (write-char #\/ out)
+     (unless (null? path)
+       (when abs? (write-char #\/ out))
+       (let loop ([path-components path])
          (match-define (path/param path params)
            (car path-components))
          (write-string
           (maybe-percent-encode
            (string-append path (string-join params ";"))
            uri-path-segment-encode)
           out)
-         (loop (cdr path-components))))
+         (unless (null? (cdr path-components))
+           (write-char #\/ out)
+           (loop (cdr path-components)))))
      (unless (null? query)
        (write-char #\? out)
        (for* ([pair (in-list query)]

http-easy-test/net/http-easy/private/url.rkt

@@ -65,7 +65,9 @@
           ("http://[email protected]:5100" . "http://[email protected]:5100")
           ("http://example.com/a/b/c" . "http://example.com/a/b/c")
           ("http://example.com/a%2Bb.mp3" . "http://example.com/a%2Bb.mp3")
-          ("http://example.com/a%2Bb.mp3?c=d+e" . "http://example.com/a%2Bb.mp3?c=d%2Be")))
+          ("http://example.com/a%2Bb.mp3?c=d+e" . "http://example.com/a%2Bb.mp3?c=d%2Be")
+          ("a/b/c" . "a/b/c")
+          ("/a/b/c" . "/a/b/c")))
 
       (for* ([pair (in-list tests)]
              [s (in-value (car pair))]