Editor: Hiragana conversion display issue

I confirmed that this also happens with ATOK and the standard Japanese IME built in to macOS.

_[Originally reported](https://github.com/Automattic/simplenote-electron/issues/909) by @naokomc_:

#### Steps to reproduce
1. Add [Google IME Japanese](https://www.google.co.jp/ime/) to your Mac
2. Select hiragana = "ひらがな (Google)" as your input method
3. Start typing "kyo" in a Simplnote note and select one of the date formats
4. You get a garbled text as a preview
5. When you actually select a date format, the confirmed text is displayed correctly.

#### OS version
macOS Sierra 10.12.6

#### Screenshot / Video

Video: https://cloudup.com/cyndeSf1ZC1

Start by reproducing the issue on macOS with Google IME Japanese, ATOK, or the standard Japanese IME, following the listed steps and observing the date-format preview. The issue does not name a source file or test, so trace the editor's handling of unconfirmed Japanese input. Done means the preview remains readable during Hiragana conversion and the confirmed date-format text remains correct.

Written by the indexing model from the issue text.