Fix XMPP API POST request

Faraday does not turn hashes into JSON by itself apparently.

app/services/ejabberd_api_client.rb

@@ -4,7 +4,7 @@ class EjabberdApiClient
   end
 
   def post(endpoint, payload)
-    res = Faraday.post("#{@base_url}/#{endpoint}", payload,
+    res = Faraday.post("#{@base_url}/#{endpoint}", payload.to_json,
                        "Content-Type" => "application/json")
 
     if res.status != 200