Encoding errors

In one notebook (en2ru.ipynb) I create function:
`def ruchar(): return 'Б'`
In another notebook I import first notebook and call ruchar()
```
from ipynb.fs.full.en2ru import *
ruchar()
```
and I get `'Р‘'`

Reproduce the encoding problem using en2ru.ipynb and the import statement `from ipynb.fs.full.en2ru import *`, then call ruchar(). Trace how the notebook importer handles the Cyrillic return value; done means the call returns `Б` rather than `Р‘`.

索引モデルが issue の本文から書いたものです。