[FIX] website_form: custom field thing blows up in P3

Custory reading seems to denote that field names are probably already
text in the normal case, and thus should not need decoding? It only
blows up in a tour so...

addons/website_form/controllers/main.py

@@ -142,7 +142,7 @@ class WebsiteForm(http.Controller):
 
             # If it's a custom field
             elif field_name != 'context':
-                data['custom'] += "%s : %s\n" % (field_name.decode('utf-8'), field_value)
+                data['custom'] += u"%s : %s\n" % (field_name, field_value)
 
         # Add metadata if enabled
         environ = request.httprequest.headers.environ