fix: 1028 - Not able to send message to slack private channel

[shashwat-appdirect]

Checklist:

• My organization is added to USERS.md.

Fixes: #1028

[CLAassistant]

All committers have signed the CLA.

[whynowy]

LGTM, thanks!

[shashwat-appdirect]

Hi @whynowy,
I tested it using curl with the latest v1.2.1, but seems that the issue is still there. And this time I am getting a different error. For public channel it is working whereas for private it is failing with below error:

2021/01/27 09:35:01 protocol error: received *http.http2GoAwayFrame before a SETTINGS frame {"level":"error","ts":1611740101.401435,"logger":"argo-events.sensor","caller":"slack/slack.go:118","msg":"unable to list channels","sensorName":"webhook","error":"Post \"https://slack.com/api/conversations.list\": connection error: PROTOCOL_ERROR","stacktrace":"github.com/argoproj/argo-events/sensors/triggers/slack.(*SlackTrigger).Execute\n\t/home/runner/work/argo-events/argo-events/sensors/triggers/slack/slack.go:118\ngithub.com/argoproj/argo-events/sensors.(*SensorContext).triggerActions\n\t/home/runner/work/argo-events/argo-events/sensors/listener.go:267\ngithub.com/argoproj/argo-events/sensors.(*SensorContext).ListenEvents.func1.3\n\t/home/runner/work/argo-events/argo-events/sensors/listener.go:158"} {"level":"error","ts":1611740101.403081,"logger":"argo-events.sensor","caller":"sensors/listener.go:160","msg":"failed to trigger actions","sensorName":"webhook","error":"failed to list channels: Post \"https://slack.com/api/conversations.list\": connection error: PROTOCOL_ERROR","errorVerbose":"Post \"https://slack.com/api/conversations.list\": connection error: PROTOCOL_ERROR\nfailed to list channels\ngithub.com/argoproj/argo-events/sensors/triggers/slack.(*SlackTrigger).Execute\n\t/home/runner/work/argo-events/argo-events/sensors/triggers/slack/slack.go:119\ngithub.com/argoproj/argo-events/sensors.(*SensorContext).triggerActions\n\t/home/runner/work/argo-events/argo-events/sensors/listener.go:267\ngithub.com/argoproj/argo-events/sensors.(*SensorContext).ListenEvents.func1.3\n\t/home/runner/work/argo-events/argo-events/sensors/listener.go:158\nruntime.goexit\n\t/opt/hostedtoolcache/go/1.14.13/x64/src/runtime/asm_amd64.s:1373","stacktrace":"github.com/argoproj/argo-events/sensors.(*SensorContext).ListenEvents.func1.3\n\t/home/runner/work/argo-events/argo-events/sensors/listener.go:160"}

Now it is not able to list the channels itself. I debugged and found that the fix which we merge might have something wrong:
We merged the below:
Types: []string{"public_channel,private_channel"},
whereas I think it should be as :
Types: []string{"public_channel","private_channel"},

Note the 2 words are the two different strings. Please suggest if my understanding is correct and we need a change. I have raised a PR for your review in case my understanding is correct - #1038

Thanks a lot!