Fix small bug where %xmode without arguments failed to run.

Added unit test.

IPython/core/tests/test_magic.py

@@ -355,3 +355,10 b' def test_cpaste():'
 
     for code in tests['fail']:
         check_cpaste(code, should_fail=True)
+
+def test_xmode():
+    # Calling xmode three times should be a no-op
+    xmode = _ip.InteractiveTB.mode
+    for i in range(3):
+        _ip.magic("xmode")
+    nt.assert_equal(_ip.InteractiveTB.mode, xmode)

IPython/core/ultratb.py

@@ -1090,7 +1090,7 b' class FormattedTB(VerboseTB, ListTB):'
         # include variable details only in 'Verbose' mode
         self.include_vars = (self.mode == self.valid_modes[2])
         # Set the join character for generating text tracebacks
-        self.tb_join_char = self._join_chars[mode]
+        self.tb_join_char = self._join_chars[self.mode]
 
     # some convenient shorcuts
     def plain(self):