unexpected behavior with Korean characters

- Have you already searched for similar issues? Please help us out and double-check first!
yes.
- Also, please don't be that person who deletes this template. It's here for a reason.
yeah.
- Thanks!

---

### Which version of React JS are you using?

✅ Officially supported ✅
- [ ] v15.4.x

⚠️ Not officially supported, expect warnings ⚠️
- [ ] v15.5.x
- [ ] v15.6.x

☣️ Not officially supported, expect warnings and errors ☣️
- [ ] v16.x.x

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- [ ] IE 9 / IE 10 / IE 11
- [ ] Edge
- [ ] Chrome

⚠️ Not officially supported, but "should work" ⚠️
- [ ] Firefox
- [ ] Safari

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### I'm submitting a ...

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- [ ] 💡 Feature Request

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> Please do not submit support request here. Instead see

> https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md

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### Issue Details

```
Please include:
- What the current behavior is
- What the desired behvaior is
- (If Bug) Steps to reproduce the issue
- (If Feature) The motivation / use case for the feature

We especially love screenshots / videos of problems, and remember
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```
Hi there.
firstly Thanks for your great work for React-Data-Grid.
I was testing a few things on your test codes in the code sand box and there's something I can't possibly understand.
please check the video I uploaded.


In my circumstances, I have to deal with Korean characters that can be typed in the RDG.
When typing in Korean and press ENTER, cell editing is not completed. (guessing...)
I do believe pressing ENTER should finish editing either typing in English or other languages.
I'm not so sure if this is a bug, please tell me what to do.
Thank you very much again.
I'll be waiting for your help.