add unit tests to show, that encoding parameter in open_resource method is necessary

tests/test_helpers.py

@@ -341,6 +341,19 @@ class TestHelpers:
         with app.open_resource("static/index.html", mode) as f:
             assert "<h1>Hello World!</h1>" in str(f.read())
 
+    @pytest.mark.skip(reason="this feature is not yet implemented")
+    @pytest.mark.parametrize("encoding", ("utf-8", 'utf-16-le'))
+    def test_open_resource_with_encoding(self, encoding):
+        app = flask.Flask(__name__)
+        if os.path.isfile('tests/static/test.txt'):
+            os.remove('tests/static/test.txt')
+
+        with open('tests/static/test.txt', 'w', encoding=encoding) as f:
+            f.write(u'' + 'Hello World!')
+
+        with app.open_resource("static/test.txt", mode='r', encoding=encoding) as f:
+            assert "Hello World!" in f.read()
+
     @pytest.mark.parametrize("mode", ("w", "x", "a", "r+"))
     def test_open_resource_exceptions(self, mode):
         app = flask.Flask(__name__)