Support to add a feed icon on pages with parameters

Followup: #2251861: Add the feed icon back to taxonomy/term/% pages

Reroll, no tests yet. One question:

+++ b/core/modules/views/lib/Drupal/views/Plugin/views/style/Rss.php
@@ -38,7 +38,7 @@ public function attachTo($display_id, $path, $title) {
-    $url = url($this->view->getUrl(NULL, $path), $url_options);
+    $url = url($this->view->getUrl(!empty($this->view->args) ? $this->view->args : NULL, $path), $url_options);

Is this needed? If NULL is passed it uses $this->args anyway.

I found that the attachment display already copies arguments and that could also work for feed displays. Here's a test.

What's the fastest way to manually verify this?

Reroll.

Re #8:
0. edit taxonomy term view feed display at /admin/structure/views/view/taxonomy_term/edit/feed_1
- below "Feed settings" is a setting "Attach to: None"
- click "None", check "Page", click Apply and Save.
1. add an article with a tag "Tag1"
2. go to /taxonomy/term/1

At the bottom of the page is a feed icon link to /taxonomy/term/all/feed with title "Subscribe to ".

3. apply this patch and reload /taxonomy/term/1

At the bottom of the page is a feed icon link to /taxonomy/term/1/feed with title "Subscribe to Tag1".

+++ b/core/modules/views/tests/modules/views_test_config/test_views/views.view.test_feed_icon.yml
@@ -23,7 +23,6 @@ display:
         type: role
         options:
           role:
-            anonymous: anonymous
             authenticated: authenticated
         provider: user

#2375107: Unable to allow multiple roles access views page

Without the patch I see core/misc/feed.png

With the patch I don't see the icon or the link to taxonomy/term/all/feed

Thanks for the details in #9 about how to test this.

Rerolled and recreated the test view.

Copied the steps from #9 to issue summary. Without this patch the link is now to taxonomy/term/*/feed which is 404 Not found.

@mgifford, Are you sure there is no link to taxonomy/term/1/feed on the term page taxonomy/term/1 with the patch after changing the "Attach to:" setting? If not, could you export and upload the view?

Works as described in #9

This issue addresses a major bug and is allowed per https://www.drupal.org/core/beta-changes. Committed f213f85 and pushed to 8.0.x. Thanks!