I created a test file

I created a new file for testing the module dataset.freeze.format.fcsv. I wrote
a test for the function ``value_to_str``.

test/test_freeze.py

@@ -0,0 +1,17 @@
+# -*- encoding: utf-8 -*-
+import unittest
+from dataset.freeze.format.fcsv import value_to_str
+
+from .sample_data import TEST_DATA
+
+
+class FreezeTestCase(unittest.TestCase):
+
+    def test_value_to_str1(self):
+        assert '2011-01-01T00:00:00' == value_to_str(TEST_DATA[0]['date'])
+
+    def test_value_to_str2(self):
+        assert 'hóla' == value_to_str(u'\u0068\u00f3\u006c\u0061')
+
+    def test_value_to_str3(self):
+        assert '' == value_to_str(None)