fixup! Add context retrieval for reads via books-mf2

src/events/fetch-context/books-mf2.js

@@ -21,7 +21,9 @@ async function fetchContext (url) {
   }
   const mf2 = await response.json()
   if (!('items' in mf2) || !mf2.items.length) return
-  return mf2.items[0].properties
+  const properties = mf2.items[0].properties
+  properties.url = [url]
+  return properties
 }
 
 module.exports = {