After changing spelling language, incorrect words not highlighted until you click outside of the message box

[jasonblais]

I confirm (by marking "x" in the [ ] below):

• This is not a troubleshooting question. Troubleshooting questions go here: http://www.mattermost.org/troubleshoot/.
• This doesn't reproduce on web browsers (such as in Chrome). If it does, issue reports go to the Mattermost Server repository.
• I have read contributing guidelines.

---

Summary
After changing spelling language, incorrect words not highlighted until you click outside of the message box

Steps to reproduce

Mac OSX: Mattermost Server v3.8.2; Mattermost Desktop App v3.7.0-RC1

1. Go to File > Settings
2. Select "Check spelling"
3. Click one of the servers, or click "x" to close the Settings page
4. Paste the following into an empty message box

Shouldn't it be easy? 1 2 3 Aimee Lindsay Russian isn't football stick Jason Eric English November screenshot helllo.

5. Right-click inside the message input box
6. Go to "Spelling Languages"
7. Select French

Expected behavior

Incorrect words highlighted right away.

Observed behavior

Incorrect words not highlighted until you click outside of the message box.

[yuya-oc]

Closed via #526