[#589] Test that config['debug'] is True, not that it exists

If it doesn't exist, consider it as False.

ckan/lib/app_globals.py

@@ -71,7 +71,7 @@ def set_main_css(css_file):
     ''' Sets the main_css using debug css if needed.  The css_file
     must be of the form file.css '''
     assert css_file.endswith('.css')
-    if ('debug' in config) and (css_file == '/base/css/main.css'):
+    if config.get('debug') and css_file == '/base/css/main.css':
         new_css = '/base/css/main.debug.css'
     else:
         new_css = css_file