HEAD requests on feed.xml sends 404 errors
### Describe the bug
HEAD requests on feed.xml send 404 errors
### Steps to reproduce
1. curl --head https://127.0.0.1:8080/@name/feed.xml
### Expected behavior
```
HTTP/1.1 200 OK
Date: Fri, 13 Nov 2020 08:56:27 GMT
Server: Apache/2.4.29 (Ubuntu)
Last-Modified: Wed, 07 Oct 2020 10:59:19 GMT
ETag: "29210-5b1129b78dc13"
Accept-Ranges: bytes
Content-Length: 168464
Vary: Accept-Encoding
Content-Type: application/xml
```
### Actual behavior
```
HTTP/2 404
server: nginx
content-type: text/html; charset=UTF-8
vary: Accept-Encoding
cache-control: no-store, max-age=0, no-cache
date: Fri, 13 Nov 2020 08:54:19 GMT
```
### Relevant logs and/or screenshots
### Context
- Castopod: all versions
- OS: all OS
- Browser: all browsers
- Web server: nginx
- Apple index is failing
### Possible fixes
in `/app/Config/Routes.php` line 436
replace
```
$routes->get('feed.xml', 'Feed/$1', ['as' => 'podcast_feed']);
```
with
```
$routes->add('feed.xml', 'Feed/$1', ['as' => 'podcast_feed']);
```
issue