Fix document check in via the API

- Closes GitLab issue #688. Thanks to inam ul haq (@inam.sys)
  for the report.
- Improve existing tests and add additional check in override tests.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/checkouts/api_views.py

@@ -77,7 +77,7 @@ class APICheckedoutDocumentView(generics.RetrieveDestroyAPIView):
     def delete(self, request, *args, **kwargs):
         document = self.get_object().document
 
-        if document.checkout_info().user == request.user:
+        if document.get_check_out_info().user == request.user:
             AccessControlList.objects.check_access(
                 obj=document, permissions=(permission_document_check_in,),
                 user=request.user