Fix: Exceptions raised in Soap/Server can leave XML entity loader
disabled.

- The original libxml_disable_entity_loader() setting was not restored
when an exception was raised in Server::_setRequest().
- Fixed by moving the libxml_disable_entity_loader(false) call a bit
further up.
- Added unittest for the intended behaviour.
Fix: Exceptions raised in Soap/Server can leave XML entity loader disabled.

- Improved the fix a bit: Instead of forcing entity loading to "false",
  the previous setting is saved and restored.
- Adjusted the unittest accordingly.
Merge branch 'hotfix/#6200-restore-libxml-entity-loader-in-soap-server'

Close #6200