plug xss hole in Markup.format()

There is currently an xss hole in Markup.format(). The failing test I added:

```
======================================================================
FAIL: test_formatting (markupsafe.tests.MarkupTestCase)
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Traceback (most recent call last):
  File "/nail/home/buck/trees/theirs/markupsafe/markupsafe/tests.py", line 78, in test_formatting
    assert actual == expected, "%r should be %r!" % (actual, expected)
AssertionError: Markup(u'<em><awesome></em>') should be '<em>&lt;awesome&gt;</em>'!

----------------------------------------------------------------------
Ran 13 tests in 0.180s
```