to_lower() method bug in Russian (cyrillic)

[exe2k]

Godot version:
3.1 beta 11

OS/device including version:
Windows 10 x64 intel i5 10gb RAM GF 660 GTX

Issue description:
to_lower() doesn't work for russian as it expected. It left almost half of letters in upper case.

Steps to reproduce:
var caps = "АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯ" print(caps.to_lower())

the result will be:

абвгдеЁжзийклмнопрстуФХЦЧШЩЪЫЬЭЮЯ
as you can see the letter "Ё" and from "Ф" to "Я" are yet capitalized.

[akien-mga]

The relevant code is in https://github.com/godotengine/godot/blob/master/core/ucaps.h
Might need extra entries for the Cyrillic letters which don't match a Latin letter.