fix issue #879 and add a test for it

Signed-off-by: Armin Ronacher <armin.ronacher@active-4.com>

flask/app.py

@@ -1492,9 +1492,9 @@ class Flask(_PackageBoundObject):
         with self._before_request_lock:
             if self._got_first_request:
                 return
-            self._got_first_request = True
             for func in self.before_first_request_funcs:
                 func()
+            self._got_first_request = True
 
     def make_default_options_response(self):
         """This method is called to create the default `OPTIONS` response.