Restore support for using pathlib.Path for static_folder.

* No longer causes AttributeError: 'PosixPath' object has no
  attribute 'rstrip'.

* This was broken by e6178fe489
  which was released in 1.1.2.

* Add a regression test that now passes.

See #3557.

src/flask/helpers.py

@@ -980,7 +980,7 @@ class _PackageBoundObject:
     @static_folder.setter
     def static_folder(self, value):
         if value is not None:
-            value = value.rstrip("/\\")
+            value = os.fspath(value).rstrip(r"\/")
         self._static_folder = value
 
     @property