internal/frontend: use full URL in redirect

Fixes golang/go#50900

Change-Id: I2ea61e8776f6c64a6c0c05f62b13ff5b9a918059
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/382160
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/frontend/details.go b/internal/frontend/details.go
index 7e6552c..13463fd 100644
--- a/internal/frontend/details.go
+++ b/internal/frontend/details.go
@@ -36,7 +36,9 @@
 		return nil
 	}
 	if strings.HasSuffix(r.URL.Path, "/") {
-		http.Redirect(w, r, strings.TrimSuffix(r.URL.Path, "/"), http.StatusMovedPermanently)
+		url := *r.URL
+		url.Path = strings.TrimSuffix(r.URL.Path, "/")
+		http.Redirect(w, r, url.String(), http.StatusMovedPermanently)
 		return
 	}