It used to return utf-8 data but was brittle in the conversion, and would fail downstream
if the calling code did not pay attention to handle utf-8 data properly.
Returning unicode forces callers to auto-cast to unicode for any string operation, saving
potential trouble later.
From the addons that currently call toxml(), none use it in a manner that would fail in case
unicode data is returned, so we should be on the safe side there too.
lp bug: https://launchpad.net/bugs/695960 fixed
bzr revid: odo@openerp.com-20110117184444-mxjeneuoa0r5a85n