Fixes updateUserGroup cannot clear the description and the default channels

[jmatsu]

Fix: https://github.com/slack-go/slack/issues/1081

Expected

We can clear the description and the default channels of usergroups by sending empty strings.

Actual

updateUserGroup ignores empty strings so this never sends empty strings to the server.

Examples

The followings are examples using curl. ref: https://api.slack.com/methods/usergroups.update

Set Up

curl -F token=... -F usergroup=SJ6JQHVBN --url 'https://slack.com/api/usergroups.update' -F description="this is a description" -F handle="zz__test_usergroup" -F name="this is a name" -F channels=C92MKCRPU {"usergroup":{"id":"SJ6JQHVBN","name":"this is a name","description":"this is a description","handle":"zz__test_usergroup","prefs":{"channels":["C92MKCRPU"],"groups":[]}}, ...}

Clear the default channels

curl -F token=... -F usergroup=SJ6JQHVBN --url 'https://slack.com/api/usergroups.update' -F description="this is a description" -F handle="zz__test_usergroup" -F name="this is a name" -F channels= {"usergroup":{"id":"SJ6JQHVBN","name":"this is a name","description":"this is a description","handle":"zz__test_usergroup","prefs":{"channels":[],"groups":[]}}, ...}

Clear the description

curl -F token=... -F usergroup=SJ6JQHVBN --url 'https://slack.com/api/usergroups.update' -F description= -F handle="zz__test_usergroup" -F name="this is a name" -F channels= {"usergroup":{"id":"SJ6JQHVBN","name":"this is a name","description":"","handle":"zz__test_usergroup","prefs":{"channels":[],"groups":[]}}, ...}

---

PR preparation

✅ Ran make pr-prop on HEAD (38367d1)

Should this be an issue instead

• [x] No ~is it a convenience method? (no new functionality, streamlines some use case)~
• [x] No ~exposes a previously private type, const, method, etc.~
• [x] No ~is it application specific (caching, retry logic, rate limiting, etc)~
• [x] No ~is it performance related.~

API changes

Since API changes have to be maintained they undergo a more detailed review and are more likely to require changes.

• ~no tests, if you're adding to the API include at least a single test of the happy case.~
  • I have added the test cases for updateUserGroup
• If you can accomplish your goal without changing the API, then do so.
  • Please let me know if you prefer a way to deprecate the current methods and create new ones.
• ~dependency changes. updates are okay. adding/removing need justification.~
  • Nothing.

Examples of API changes that do not meet guidelines:

N/A

[jmatsu]

@kanata2 Thanks for your review. Could you please have a look again?

[kanata2]

Released! https://github.com/slack-go/slack/releases/tag/v0.11.2