Update serializers.py

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

rest_framework/serializers.py

@@ -757,6 +757,9 @@ class ModelSerializer(Serializer):
                     field.read_only = True
 
                 ret[accessor_name] = field
+        
+        #Ensure that 'read_only_fields is an iterable
+        assert isinstance(self.opts.read_only_fields, (list, tuple)), '`read_only_fields` must be a list or tuple' 
 
         # Add the `read_only` flag to any fields that have bee specified
         # in the `read_only_fields` option
@@ -771,7 +774,10 @@ class ModelSerializer(Serializer):
                 "on serializer '%s'." %
                 (field_name, self.__class__.__name__))
             ret[field_name].read_only = True
-
+        
+        # Ensure that 'write_only_fields' is an iterabe
+        assert isinstance(self.opts.write_only_fields, (list, tuple)), '`read_only_fields` must be a list or tuple' 
+        
         for field_name in self.opts.write_only_fields:
             assert field_name not in self.base_fields.keys(), (
                 "field '%s' on serializer '%s' specified in "