Fix UnicodeEncodeError for non-Latin characters (#6715)

redash/destinations/slack.py

@@ -50,7 +50,7 @@ def notify(self, alert, query, user, new_state, app, host, metadata, options):
         payload = {"attachments": [{"text": text, "color": color, "fields": fields}]}
 
         try:
-            resp = requests.post(options.get("url"), data=json_dumps(payload), timeout=5.0)
+            resp = requests.post(options.get("url"), data=json_dumps(payload).encode("utf-8"), timeout=5.0)
             logging.warning(resp.text)
             if resp.status_code != 200:
                 logging.error("Slack send ERROR. status_code => {status}".format(status=resp.status_code))