Auto merge of #28634 - negator:patch-1, r=jdm

Preserve URI fragment across redirects

https://www.rfc-editor.org/rfc/rfc7231#section-7.1.2

```
If the Location value provided in a 3xx (Redirection) response does
   not have a fragment component, a user agent MUST process the
   redirection as if the value inherits the fragment component of the
   URI reference used to generate the request target (i.e., the
   redirection inherits the original reference's fragment, if any).
```

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components/net/http_loader.rs

@@ -831,7 +831,7 @@ pub fn http_fetch(
         }
 
         // Substep 2-3.
-        let location = response
+        let mut location = response
             .actual_response()
             .headers
             .get(header::LOCATION)
@@ -845,6 +845,12 @@ pub fn http_fetch(
             });
 
         // Substep 4.
+        if let Some(Ok(ref mut location)) = location {
+            if location.fragment().is_none() {
+                let current_url = request.current_url();
+                location.set_fragment(current_url.fragment());
+            }
+        }
         response.actual_response_mut().location_url = location;
 
         // Substep 5.