Upwrap results of from_utf8* for 0.10-pre.

It might be better than what I did to match with Option and fail! if it
is None, because of error messages.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #4 from omasanori/unwrap

Upwrap results of from_utf8* for 0.10-pre.