"Python" -> "Pythoん": "n" at the end of an English word becomes ん

[DarkTrick]

Issue description:
-------------------
"n" at the end of an English word is not processed as expected.

Steps to reproduce:
--------------------
    - Prepare a random input field
    - Got to JP input mode
    - Type "Python"
    - Press Enter

Expected:
---------
"Python" will be displayed.

Acutal:
--------
"Pythoん" will be displayed.

Can you reproduce your problem when you restart ibus-daemon? (yes / no):
-------------------------------------------------------------------------
yes

Do you see any errors when you run ibus-daemon with the verbose option?:
-------------------------------------------------------------------------
no

Misc:
-------
- Input Mode: Hiragana, Full-Width
- `Switch "Hiragana" and "Latin" with Shift key...` is activated

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ibus-anthy 1.5.11-1build1
ProcVersionSignature: Ubuntu 5.4.0-45.49-generic 5.4.55
Uname: Linux 5.4.0-45-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.8
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: XFCE
Date: Sun Sep 6 10:10:48 2020
SourcePackage: ibus-anthy
UpgradeStatus: Upgraded to focal on 2020-07-24 (43 days ago)