Updating slack-message with multiple channels only updates one channel

### Your checklist for this issue

- [x] Jenkins version: 2.222.4

- [x] Plugin version: 2.40

- [x] OS: ubuntu 16.04 LTS

### Description

When posting message to multiple channels ('channels: "#mychannel, #secondchannel") messages work as expected. But when return-value is stored to variable and using the update-message feature (channels: previous_message.channel_id, timestamp: previous_message.ts), only the first channel message gets updated.

Start from the update-message feature and the handling of previous_message.channel_id and previous_message.ts. Reproduce a message sent to multiple channels, then update it using the returned values. Done means the update reaches every channel message rather than only the first.

Written by the indexing model from the issue text.