[FIX] Fix account_budget name unicode error (7.0 lp:1292245)

Fix bug https://bugs.launchpad.net/openobject-addons/+bug/1292245:
    Invoice budget's warning when crossovered budget lines are not related
    to any account will cause a UnicodeEncodeError if the account has
    unicode-only characters such as é.

Make sure that the budget name is a unicode to avoid UnicodeEncodeError
which happens when budget name contains UTF-8 characters.