Dumps Dict values in UTF-8 instead of ASCII

This is how they are stored in the database and so we must use also the UTF-8
version when using unaccent to properly remove the accents.

issue11418
(grafted from db29f8a10314f48f68be5b68ec075ae4f37edb2f)

--HG--
branch : 6.0

trytond/model/fields/dict.py

@@ -14,7 +14,8 @@ from .field import Field, SQL_OPERATORS
 
 # Use canonical form
 dumps = partial(
-    json.dumps, cls=JSONEncoder, separators=(',', ':'), sort_keys=True)
+    json.dumps, cls=JSONEncoder, separators=(',', ':'), sort_keys=True,
+    ensure_ascii=False)
 
 
 class ImmutableDict(dict):