Make tags and cats case insensitive. Fixes #704.

More precisely, group tags or categories without considering the case.
This fixes the bug where two categories with just the case as difference were
considered as distinct, but generate the same file: one overwriting the other.

Thanks to @Avaris for helping with the tests.

pelican/tests/test_generators.py

@@ -89,17 +89,29 @@ class TestArticlesGenerator(unittest.TestCase):
             ['This is an article without category !', 'published',
              'TestCategory', 'article'],
             ['This is a super article !', 'published', 'yeah', 'article'],
-            ['マックOS X 10.8でパイソンとVirtualenvをインストールと設定', 'published', '指導書', 'article']
+            ['マックOS X 10.8でパイソンとVirtualenvをインストールと設定',
+             'published', '指導書', 'article']
         ]
         self.assertEqual(sorted(articles_expected), sorted(articles))
 
     def test_generate_categories(self):
 
         generator = self.get_populated_generator()
+        # test for name
+        # categories are grouped by slug; if two categories have the same slug
+        # but different names they will be grouped together, the first one in
+        # terms of process order will define the name for that category
         categories = [cat.name for cat, _ in generator.categories]
-        categories_expected = ['Default', 'TestCategory', 'Yeah', 'test',
-                               'yeah', '指導書']
-        self.assertEqual(categories, categories_expected)
+        categories_alternatives = (
+            sorted(['Default', 'TestCategory', 'Yeah', 'test', '指導書']),
+            sorted(['Default', 'TestCategory', 'yeah', 'test', '指導書']),
+        )
+        self.assertTrue(sorted(categories) in categories_alternatives)
+        # test for slug
+        categories = [cat.slug for cat, _ in generator.categories]
+        categories_expected = ['default', 'testcategory', 'yeah', 'test',
+                               'zhi-dao-shu']
+        self.assertEqual(sorted(categories), sorted(categories_expected))
 
     def test_do_not_use_folder_as_category(self):
 
@@ -113,9 +125,20 @@ class TestArticlesGenerator(unittest.TestCase):
                             CUR_DIR, _DEFAULT_CONFIG['THEME'], None,
                             _DEFAULT_CONFIG['MARKUP'])
         generator.generate_context()
-
+        # test for name
+        # categories are grouped by slug; if two categories have the same slug
+        # but different names they will be grouped together, the first one in
+        # terms of process order will define the name for that category
         categories = [cat.name for cat, _ in generator.categories]
-        self.assertEqual(categories, ['Default', 'Yeah', 'test', 'yeah', '指導書'])
+        categories_alternatives = (
+            sorted(['Default', 'Yeah', 'test', '指導書']),
+            sorted(['Default', 'yeah', 'test', '指導書']),
+        )
+        self.assertTrue(sorted(categories) in categories_alternatives)
+        # test for slug
+        categories = [cat.slug for cat, _ in generator.categories]
+        categories_expected = ['default', 'yeah', 'test', 'zhi-dao-shu']
+        self.assertEqual(sorted(categories), sorted(categories_expected))
 
     def test_direct_templates_save_as_default(self):