[FIX] render_attribute should return utf-8 encoded string
    openerp/addons/base/ir/ir_qweb.py", line 282, in render_node
        generated_attributes += self.render_attribute(element, att, val, qwebcontext)
        QWebException: 'ascii' codec can't decode byte 0xc3 in position 29: ordinal not in range(128)
    exception when try to concate utf-8 encoded string(generated_attributes) to unicode