Keep file name and query for fragment-only redirect Location

Found while testing crates/fetch/src/http.rs:139 (Url::join)

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/fetch/src/http.rs

@@ -135,6 +135,10 @@ impl Url {
         if l.starts_with('/') {
             return Url::parse(&format!("{base}{l}"));
         }
+        // empty or fragment-only: the same document
+        if l.is_empty() || l.starts_with('#') {
+            return Ok(self.clone());
+        }
         // relative to the current directory
         let dir = self.path.split('?').next().unwrap_or("/");
         let dir = dir.rsplit_once('/').map_or("", |(d, _)| d);
@@ -612,6 +616,15 @@ mod tests_connect {
 mod tests {
     use super::*;
 
+    #[test]
+    fn fragment_only_or_empty_location_keeps_file_name_and_query() {
+        let base = Url::parse("https://a.example/x/y/file?q").unwrap();
+        for l in ["#top", "", "  "] {
+            let u = base.join(l).unwrap();
+            assert_eq!((u.tls, u.host.as_str(), u.port, u.path.as_str()), (true, "a.example", 443, "/x/y/file?q"), "{l:?}");
+        }
+    }
+
     #[test]
     fn rejects_oversized_and_control_character_urls() {
         assert!(Url::parse(&format!("https://example.com/{}", "x".repeat(16 * 1024))).is_err());