PYTHON-621: Fixed GridOut.read(0) returning string in Python 3

gridfs/grid_file.py

@@ -467,7 +467,7 @@ class GridOut(object):
         self._ensure_file()
 
         if size == 0:
-            return ""
+            return EMPTY
 
         remainder = int(self.length) - self.__position
         if size < 0 or size > remainder:

test/test_grid_file.py

@@ -73,6 +73,9 @@ class TestGridFile(unittest.TestCase):
         g = GridOut(self.db.fs, f._id)
         self.assertEqual(b(""), g.read())
 
+        # test that reading 0 returns proper type
+        self.assertEqual(b(""), g.read(0))
+
     def test_md5(self):
         f = GridIn(self.db.fs)
         f.write(b("hello world\n"))