Fix saml_attr error

Author: OskarPersson

djangosaml2/backends.py

@@ -113,7 +113,7 @@ def get_attribute_value(self, django_field, attributes, attribute_mapping):
         logger.debug('attribute_mapping: %s', attribute_mapping)
         for saml_attr, django_fields in attribute_mapping.items():
             if django_field in django_fields and saml_attr in attributes:
-                saml_user = attributes.get('saml_attr', [None])[0]
+                saml_user = attributes.get(saml_attr, [None])[0]
                 if not saml_user:
                     logger.error('attributes[saml_attr] attribute '
                                  'value is missing. Probably the user '