Add support for PathLike objects in static file helpers

See: https://www.python.org/dev/peps/pep-0519/

This is mostly encountered with pathlib in python 3, but this API
suggests any PathLike object can be treated like a filepath with
`__fspath__` function.

tests/test_helpers.py

@@ -36,6 +36,19 @@ def has_encoding(name):
         return False
 
 
+class FakePath(object):
+    """Fake object to represent a ``PathLike object``.
+
+    This represents a ``pathlib.Path`` object in python 3.
+    See: https://www.python.org/dev/peps/pep-0519/
+    """
+    def __init__(self, path):
+        self.path = path
+
+    def __fspath__(self):
+        return self.path
+
+
 class FixedOffset(datetime.tzinfo):
     """Fixed offset in hours east from UTC.
 
@@ -527,6 +540,15 @@ class TestSendfile(object):
         assert 'x-sendfile' not in rv.headers
         rv.close()
 
+    def test_send_file_pathlike(self, app, req_ctx):
+        rv = flask.send_file(FakePath('static/index.html'))
+        assert rv.direct_passthrough
+        assert rv.mimetype == 'text/html'
+        with app.open_resource('static/index.html') as f:
+            rv.direct_passthrough = False
+            assert rv.data == f.read()
+        rv.close()
+
     @pytest.mark.skipif(
         not callable(getattr(Range, 'to_content_range_header', None)),
         reason="not implemented within werkzeug"
@@ -681,6 +703,12 @@ class TestSendfile(object):
         assert cc.max_age == 3600
         rv.close()
 
+        # Test with static file handler.
+        rv = app.send_static_file(FakePath('index.html'))
+        cc = parse_cache_control_header(rv.headers['Cache-Control'])
+        assert cc.max_age == 3600
+        rv.close()
+
         class StaticFileApp(flask.Flask):
             def get_send_file_max_age(self, filename):
                 return 10
@@ -706,6 +734,14 @@ class TestSendfile(object):
         assert rv.data.strip() == b'Hello Subdomain'
         rv.close()
 
+    def test_send_from_directory_pathlike(self, app, req_ctx):
+        app.root_path = os.path.join(os.path.dirname(__file__),
+                                     'test_apps', 'subdomaintestmodule')
+        rv = flask.send_from_directory(FakePath('static'), FakePath('hello.txt'))
+        rv.direct_passthrough = False
+        assert rv.data.strip() == b'Hello Subdomain'
+        rv.close()
+
     def test_send_from_directory_bad_request(self, app, req_ctx):
         app.root_path = os.path.join(os.path.dirname(__file__),
                                      'test_apps', 'subdomaintestmodule')