[FIX] web: encoding css on multi page

In the small cases where utf-8 is not escaped in the CSS of a module*, an error
could happen when breaking minified CSS on multiple page (for IE9).

For the issue #5050

*currenlty for 8.0 : https://gist.github.com/nle-odoo/e353b22f89031ced21a5

openerp/addons/base/ir/ir_qweb.py

@@ -1219,7 +1219,7 @@ class AssetsBundle(object):
         if page_number:
             return content
         if content is None:
-            css = self.css()
+            css = self.css().decode('utf-8')
             re_rules = '([^{]+\{(?:[^{}]|\{[^{}]*\})*\})'
             re_selectors = '()(?:\s*@media\s*[^{]*\{)?(?:\s*(?:[^,{]*(?:,|\{(?:[^}]*\}))))'
             css_url = '@import url(\'/web/css.%%d/%s/%s\');' % (self.xmlid, self.version)