fixing smaller problems in apprise and mattermost

lib/notification/adapter/mattermost.js

@@ -13,10 +13,10 @@ export const send = ({ serviceName, newListings, notificationConfig, jobKey }) =
   return fetch(webhook, {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },
-    body: {
+    body: JSON.stringify({
       channel: channel,
       text: message,
-    },
+    }),
   });
 };
 export const config = {