[FIX] base: ir.qweb: ensure element.tail is correctly encoded

element.{text,tail} are either ascii-compatible `str`, or `unicode`
when non-ascii-compatible. This could force the implicit decoding
of utf-8 encoded contents when joining template bits, breaking
the rendering.

Fixes #1085, and related to #1130
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64