Add test capturing failure. Ref #486.

setuptools/tests/test_unicode_utils.py

+from setuptools import unicode_utils
+
+
+def test_filesys_decode_fs_encoding_is_None(monkeypatch):
+    """
+    Test filesys_decode does not raise TypeError when
+    getfilesystemencoding returns None.
+    """
+    monkeypatch.setattr('sys.getfilesystemencoding', lambda: None)
+    unicode_utils.filesys_decode(b'test')