Change `set.append` -> `set.add` in `headerid.unique`

Fixes #195. This was getting missed because the HeadrerId extension's
reset method was resetting the IDs to a list. However, some third party
extensions may want to call the unique function and it should work as
documented. Interestingly, the TOC extension was using it and passing in
a list as well. All fixed now. Also added a test of the `unique` function
directly so we shouldn't repeat this in the future.

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/test_extensions.py b/tests/test_extensions.py
index fa9a801..d9d77b8 100644
--- a/tests/test_extensions.py
+++ b/tests/test_extensions.py
@@ -199,6 +199,13 @@ class TestHeaderId(unittest.TestCase):
         self.assertEqual(self.md.convert(text),
             '<h1 id="some-header">Some Header</h1>')
 
+    def testUniqueFunc(self):
+        """ Test 'unique' function. """
+        from markdown.extensions.headerid import unique
+        ids = set(['foo'])
+        self.assertEqual(unique('foo', ids), 'foo_1')
+        self.assertEqual(ids, set(['foo', 'foo_1']))
+
     def testUniqueIds(self):
         """ Test Unique IDs. """