Fix python 2 support for StringIO.

dataset/freeze/format/fcsv.py

@@ -25,7 +25,6 @@ class CSVSerializer(Serializer):
     def write(self, path, result):
         keys = list(result.keys())
         if path not in self.handles:
-
             # handle fileobj that has been passed in:
             if path is not None:
                 if PY3:  # pragma: no cover
@@ -39,7 +38,7 @@ class CSVSerializer(Serializer):
             if PY3:  # pragma: no cover
                 writer.writerow(keys)
             else:
-                writer.writerow([k.encode('utf-8') for k in keys])
+                writer.writerow([value_to_str(k) for k in keys])
             self.handles[path] = (writer, fh)
         writer, fh = self.handles[path]
         values = [value_to_str(result.get(k)) for k in keys]

test/test_freeze.py

@@ -63,10 +63,13 @@ class FreezeTestCase(unittest.TestCase):
             fh.close()
 
     def test_memory_streams(self):
-        import io
+        if PY3:
+            from io import StringIO
+        else:
+            from io import BytesIO as StringIO
 
         for fmt in ('csv', 'json', 'tabson'):
-            with io.StringIO() as fd:
+            with StringIO() as fd:
                 freeze(self.tbl.all(), format=fmt, fileobj=fd)
                 self.assertFalse(fd.closed, 'fileobj was closed for format %s' % fmt)
                 fd.getvalue()  # should not throw