defaultValue for favicon should be "/favicon.ico" rather than "favicon.ico"

[darinwf]

According to the HTML spec (https://html.spec.whatwg.org/#rel-icon), the fallback favicon should be "/favicon.ico" resolved against the document URL.

However, parser.js falls back to resolving "favicon.ico" against the document URL, which can result in some favicon URLs that are unexpected.

The fix is pretty trivial:

--- parser.js	2021-08-30 13:01:51.000000000 -0700
+++ parser.js-new	2021-08-30 16:02:05.000000000 -0700
@@ -94,7 +94,7 @@
         }
       }
     ],
-    defaultValue: (context) => 'favicon.ico',
+    defaultValue: (context) => '/favicon.ico',
     processors: [
       (icon_url, context) => makeUrlAbsolute(context.url, icon_url)
     ]