Add test for 'txn.user = None'.

Restores 100% coverage.

transaction/_transaction.py

@@ -138,6 +138,8 @@ class Transaction(object):
     def user(self, v):
         if v is not None:
             self._user = text_or_warn(v)
+        else:
+            self._user = u""
 
     @property
     def description(self):

transaction/tests/test__transaction.py

@@ -1060,6 +1060,12 @@ class TransactionTests(unittest.TestCase):
         txn.setUser(u'phreddy', u'/bedrock')
         self.assertEqual(txn.user, u'/bedrock phreddy')
 
+    def test_user_w_none(self):
+        txn = self._makeOne()
+        txn.user = b'phreddy'
+        txn.user = None  # resets to empty text
+        self.assertEqual(txn.user, u'')
+
     def _test_user_non_text(self, user, path, expect, both=False):
         txn = self._makeOne()
         with warnings.catch_warnings(record=True) as w: