text: Title: Import Columns wich dont dont exist in the Model fields (read only not working correctly ?)
  Body: i got this Resource:
  
  ```
  class ProductionResource(resources.ModelResource):
      article = fields.Field(column_name="GMC", attribute="article", widget=ForeignKeyWidget(Article, "gmc"))
      amount = fields.Field(column_name="Menge", attribute="amount")
      number = fields.Field(column_name="Lagerplatz", attribute="number")
      sheet = fields.Field(column_name="Bogen", readonly=True)
      use = fields.Field(column_name="Nutzen", readonly=True)
      version = fields.Field(column_name="Version", readonly=True, widget=CharWidget())
      comment = fields.Field(column_name="Bemerkung", readonly=True)
  
      def before_import_row(self, row, row_number=None, **kwargs):
          row["Lagerplatz"] = f"PRODUKTION ({row['GMC']})"
  
      def after_import_row(self, row, row_result, row_number=None, **kwargs):
          Movement.objects.create(
              article_number=row["GMC"],
              storage_place_barcode=row["Lagerplatz"],
              amount_moved=row["Menge"],
              operation=1,
              purpose=5,
              user=kwargs['user'],
              comment=f"({row['Bogen']}/{row['Nutzen']}) {row['Version']} {row['Bemerkung']}"
          )
  
      class Meta:
          model = StoragePlace
          use_bulk = True
          use_transactions = True
          skip_unchanged = False
          report_skipped = True
          import_id_fields = ["article", "number"]
          exclude = ["id", "blocked", "can_store_multiple_articles"]
  ```
  
  The fields sheet, use, version and comment don't exist and this is the reason why i get key errors. I thought the read only attribute would solve my problem but apparently it dose not.