* move conversion of data to json in slack API handling
at one point in do_notify_slack, we do operations on the payload
variable assuming it's a dict, but it's not: it's a json encoded string.
it's useful to operate on the payload as a dict rather than a string, so
solve this problem by moving the jsonify call to right before sending
the payload to the slack API.
fixes#1097
* add changelog fragment
* Update changelogs/fragments/1101-slack-ts-fix.yaml
Co-authored-by: Felix Fontein <felix@fontein.de>
* return payload as a json encoded string for backwards compatibility
Co-authored-by: Felix Fontein <felix@fontein.de>
