Fix test item nesting for non-Function tests. (#67)

The original code assumed that all test items were subclasses of "Function",
but they aren't.  The easy case is PyFlakes tests, but there are many more
examples.

So use an explicit sentinel instead of assuming.

Fixes: #66

Author: kleptog

pytest_reportportal/service.py

@@ -112,6 +112,7 @@ def collect_tests(self, session):
             parts = self._get_item_parts(item)
             # Add all parts in revers order to parts_out
             parts_out.extend(reversed(parts))
+            parts_out.append(None)   # marker
             while parts:
                 part = parts.pop(0)
                 if part in self._start_stack:
@@ -175,9 +176,9 @@ def finish_pytest_item(self, status, issue=None):
         self.RP.finish_test_item(**fta_rq)
 
         while self._finish_stack:
-            if isinstance(self._finish_stack[0], Function):
-                break
             part = self._finish_stack.pop(0)
+            if part is None:
+                break
             if self._finish_stack.count(part):
                 continue
             payload = {