Update serializers.py

Prevent iterating over a string that is supposed to be an iterable <==> Prevent read_only_fields = ('some_string)
Update serializers.py

Prevent iterating over a string that is supposed to be an iterable <==> Prevent read_only_fields = ('some_string)
Update serializers.py
Merge pull request #1474 from elmehdikarami/master

Update serializers.py