[PATCH 0 of 1] minirst: use unicode string as intermediate form for replacement

[FUJIWARA Katsunori]

# this is re-send with correct "From" header.
# sorry if you have already received same one.

Current stable version often fails to show help documents in CP932
encoding , known as Japanese Shift-JIS.

This issue was fixed by changeset 521c8e0c93bf at once, but re-opened
by 0ad0ebe67815.

This patch redones part of 521c8e0c93bf (with comment), and adds test
to prevent from accidental re-open of the issue.