Fix bridge no longer POSTing username and avatar (slack) (#536)

* Fixed pointer/reference issue in populateUsers. [#536]

* Accepted codestyle suggestion.

* Update bridge/slack/helpers.go

Co-Authored-By: patcon <patrick.c.connolly@gmail.com>

* Update helpers.go

bridge/slack/helpers.go

@@ -65,8 +65,10 @@ func (b *Bslack) populateUsers() {
 	}
 
 	newUsers := map[string]*slack.User{}
-	for _, user := range users {
-		newUsers[user.ID] = &user
+	for i := range users {
+		// Use array index for pointer, not the copy
+		// See: https://stackoverflow.com/a/29498133/504018
+		newUsers[users[i].ID] = &users[i]
 	}
 
 	b.usersMutex.Lock()