--settings=custom_settings.py is not overriding all settings with custom settings

``--settings=custom_settings.py`` is not overriding all settings with custom settings.

Eg. ``CHROME_START_WIDTH`` was not updated when set inside a custom settings file.

Here's the full list of settings that were not modified as expected:
``CHROME_START_WIDTH``
``CHROME_START_HEIGHT``
``HEADLESS_START_WIDTH``
``HEADLESS_START_HEIGHT``
