Make sure locale is what we want before/after the tests

The locale is a global state, and it was not properly reset to
whatever it was before the unitttest possibly changed it.
This is now fixed.

Not restoring the locale led to weird issues: depending on
the order chosen by "python -m unittest discover" to run
the unit tests, some tests would apparently randomly fail
due to the locale not being what was expected.

For example, test_period_in_timeperiod_archive would
call mock('posts/1970/ 1月/index.html',...) instead of
expected mock('posts/1970/Jan/index.html',...) and fail.

pelican/tests/test_contents.py

@@ -4,6 +4,7 @@ from __future__ import unicode_literals, absolute_import
 import six
 from datetime import datetime
 from sys import platform
+import locale
 
 from pelican.tests.support import unittest, get_settings
 
@@ -22,6 +23,8 @@ class TestPage(unittest.TestCase):
 
     def setUp(self):
         super(TestPage, self).setUp()
+        self.old_locale = locale.setlocale(locale.LC_ALL)
+        locale.setlocale(locale.LC_ALL, str('C'))
         self.page_kwargs = {
             'content': TEST_CONTENT,
             'context': {
@@ -35,6 +38,9 @@ class TestPage(unittest.TestCase):
             'source_path': '/path/to/file/foo.ext'
         }
 
+    def tearDown(self):
+        locale.setlocale(locale.LC_ALL, self.old_locale)
+
     def test_use_args(self):
         # Creating a page with arguments passed to the constructor should use
         # them to initialise object's attributes.