Add a check for a custom role key - specified via an optional project...

Add a check for a custom role key - specified via an optional project parameter - and merge into the existing list of roles if present.

django_auth_lti/middleware.py

 from django.contrib import auth
 
-from django.core.exceptions import ImproperlyConfigured, PermissionDenied
+from django.core.exceptions import ImproperlyConfigured
+from django.conf import settings
 
 from timer import Timer
 
@@ -92,6 +93,9 @@ class LTIAuthMiddleware(object):
                     'custom_canvas_user_login_id': request.POST.get('custom_canvas_user_login_id', None),
                     'roles': request.POST.get('roles', '').split(','),
                 }
+                # If a custom role key is defined in project, merge into existing role list
+                if getattr(settings, 'LTI_CUSTOM_ROLE_KEY', None):
+                    lti_launch['roles'] += request.POST.get(settings.LTI_CUSTOM_ROLE_KEY, '').split(',')
 
                 request.session['LTI_LAUNCH'] = lti_launch