[FIX] OPW 577359: encoding issues when fetching mails with inconsistent encodings
[FIX] OPW 577359: encoding issues when fetching mails with inconsistent encodings

By giving control on the error handling in
tool.ustr(), we let the caller decide what
should happen with invalid characters.
e.g. with errors="replace", all invalid
characters will be replaced by the UTF-8
question-mark character U+FFFD.

akretion/openerp-server,akretion/openerp-server,akretion/openerp-server