Fix typo in in XmlImporte._check_if_namespace_meta_information_is_added which results in an exception.

In case it was required after the import to still add an ns an exception occured. On normal flow the line `_check_if_namespace_meta_information_is_added`s  with the typo isn't executed and no test was available to test the behaviour.

* Added test for `_check_if_namespace_meta_information_is_added`
* Fix the type