Fix Web Push notification delivery

Finch does not automatically append header content-type: octet-stream for binary payloads.

lib/pleroma/http/web_push.ex

@@ -6,7 +6,11 @@ defmodule Pleroma.HTTP.WebPush do
   @moduledoc false
 
   def post(url, payload, headers, options \\ []) do
-    list_headers = Map.to_list(headers)
+    list_headers =
+      headers
+      |> Map.to_list()
+      |> Kernel.++([{"content-type", "octet-stream"}])
+
     Pleroma.HTTP.post(url, payload, list_headers, options)
   end
 end