Japanese input bug

[okano4413]

What does it do?

Fix bug related on convertible character input issue like Japanese.

Why is it needed?

I think , some users who using CJK input are in trouble by this behavior.

How to test it?

How to reproduce?

1. Build example project of flutter_chat_ui with web browser.
2. Enter Japanese characters by Roman alphabets. for example, こんにちは(ko nn ni ti ha)
3. If you entered above characters, you can see "kおんにちは" (Japanese input is wrong in Flutter web. #476 )
4. Press enter key once, then "kおんにちは" is sent as message, and "おんにちは" remains in text field.( Remain characters being converted when press enter key with Japanese input. #514)

My commit fix these issues.

Related issues/PRs

#514
#476

[demchenkoalex]

Thanks for the PR!