Fixes error when Slack API returns errors as strings instead of objects:
"json: cannot unmarshal string into Go struct field chatResponseFull.SlackResponse.errors of type map[string]interface{}"
The bug was in SlackResponseErrors.UnmarshalJSON which didn't handle
string error values. This was fixed in slack-go/slack v0.17.2+.
Added test case to verify the fix works.
Lukasz Jagiello