ASSETS_DEBUG should fallback to Django DEBUG by default

[IlyaSemenov]

Steps to reproduce

1. Set DEBUG = True, do not set ASSETS_DEBUG
2. Do not run manage.py collectstatic
3. Add this to template: {% assets "admin/js/vendor/jquery/jquery.js", output="jquery.js" %}{% endassets %}
4. django-assets will crash with BundleError (file does not exist)
5. Set ASSETS_DEBUG = True
6. Everything will work fine.

Expected

Step 4. Everything works fine.

In other words, if ASSETS_DEBUG is not set explicitly, django-assets should follow Django DEBUG setting (and use staticfiles finders). Right now we are forced to change these settings simultaneously.

[IlyaSemenov]

#62 is referring the same issue, actually.

[IlyaSemenov]

and #67 provides the PR... 🤦