Allow to disable a downloader handler.

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.