Fix FSEntry's path type handling

From the comments I understood that ``FSEntry`` aims to store the ``path``
using the binary form. This commit addresses two issues:

* In Py2, ``FSEntry`` was encoding ``path`` only with the `ascii` encoder
  raising ``UnicodeEncodeError`` when encoding certain Unicode strings.
* In Py3, it was not converting to bytes.

metsrw/fsentry.py

@@ -105,10 +105,9 @@ def __init__(self, path=None, label=None, use='original', type=u'Item',
         # path can validly be any encoding; if this value needs
         # to be spliced later on, it's better to treat it as a
         # bytestring than as actually being encoded text.
-        # TODO update this with six and bytes
-        if path:
-            path = str(path)
         self.path = path
+        if isinstance(self.path, six.text_type):
+            self.path = self.path.encode('utf-8')
         if label is None and path is not None:
             label = os.path.basename(path)
         self.label = label

tests/test_fsentry.py

@@ -2,13 +2,21 @@
 import pytest
 from unittest import TestCase
 import uuid
+import six
 
 import metsrw
 
 
 class TestFSEntry(TestCase):
     """ Test FSEntry class. """
 
+    def test_path_is_binary(self):
+        """It should store the ``path`` as a bytestring."""
+        assert isinstance(
+            metsrw.FSEntry(u'💜🎑💜', type='Directory').path, six.binary_type)
+        assert isinstance(
+            metsrw.FSEntry(b'💜🎑💜', type='Directory').path, six.binary_type)
+
     def test_create_invalid_checksum_type(self):
         """ It should only accept METS valid checksum types. """
         metsrw.FSEntry(