fix(test_index): fix encoding

I really never want to touch python again, and never deal with
py2/3 unicode handling anymore.
By now, it seems pretty much anything is better.
Is python to be blamed for it entirely ?
Probably not, but there are better alternatives.
Nim ? Rust ? Ruby ?
Totally

Author: Byron

git/test/test_index.py

@@ -787,8 +787,8 @@ def test_index_bare_add(self, rw_bare_repo):
     def test_add_utf8P_path(self, rw_dir):
         # NOTE: fp is not a Unicode object in python 2 (which is the source of the problem)
         fp = os.path.join(rw_dir, 'ø.txt')
-        with open(fp, 'w') as fs:
-            fs.write('content of ø')
+        with open(fp, 'wb') as fs:
+            fs.write(u'content of ø'.encode('utf-8'))
 
         r = Repo.init(rw_dir)
         r.index.add([fp])