Alow to disable a downloader handler just like any other component.

Scrapy allows to disable any component-like setting (i.e. extension, middlewares) by setting the value to None. But this doesn't work for the setting DOWNLOAD_HANDLERS as it doesn't use the helper build_component_list because strictly speaking is doesn't follow the the components definition format.

A solution might be adding an additional helper to handle the components-like setting definition without weights. But, as the DOWNLOAD_HANDLERS setting is the only one needing this, it's not enough motivation to add more code than just skipping the None values within the DownloadHandlers class.