[FIX] export: decode Field strings before comparisons

Allows exporting fields whose string is a non-ascii *byte*string (rather than unicode).

backport of 7286f4e424 fixing #773 to branch 7.0 by request of @flotho

addons/web/controllers/main.py

@@ -1481,7 +1481,7 @@ class Export(openerpweb.Controller):
             fields['.id'] = fields.pop('id', {'string': 'ID'})
 
         fields_sequence = sorted(fields.iteritems(),
-            key=lambda field: field[1].get('string', ''))
+            key=lambda field: openerp.tools.ustr(field[1].get('string', '')))
 
         records = []
         for field_name, field in fields_sequence: