 Some items were not contained in blocks AssertionError
The code below causes the DataFrame internals to get confused and throws an AssertionError on `print df.values`. The code looks contrived, but its just a simplified version of something I was trying to do. It doesn't seem to matter whether any columns are actually renamed so I simplified that also, but the rename calls do contribute to the problem.

```
import pandas as pd
df = pd.DataFrame({'a': [1, 2],
                   'b': [3, 4],
                   'c': [5, 6]})

df = df.set_index(['a', 'b'])
df = df.unstack()
df = df.rename(columns={'a': 'd'})
df = df.reset_index()
print df._data
df = df.rename(columns={})
print df._data
print df.values

```

```
BlockManager
Items: MultiIndex
[(u'a', u''), (u'c', 3), (u'c', 4)]
Axis 1: Int64Index([0, 1], dtype=int64)
FloatBlock: [(c, 3), (c, 4)], 2 x 2, dtype float64
IntBlock: [(a, )], 1 x 2, dtype int64

BlockManager
Items: MultiIndex
[(u'a', u''), (u'c', 3), (u'c', 4)]
Axis 1: Int64Index([0, 1], dtype=int64)
FloatBlock: [(a, ), (c, 3)], 2 x 2, dtype float64
IntBlock: [(a, )], 1 x 2, dtype int64

Traceback (most recent call last):
  File "/home/mtk/bug.py", line 13, in <module>
    print df.values
  File "/Users/mtk/Source/pandas/pandas/core/frame.py", line 1779, in as_matrix
    return self._data.as_matrix(columns).T
  File "/Users/mtk/Source/pandas/pandas/core/internals.py", line 1513, in as_matrix
    mat = self._interleave(self.items)
  File "/Users/mtk/Source/pandas/pandas/core/internals.py", line 1549, in _interleave
    raise AssertionError('Some items were not contained in blocks')
AssertionError: Some items were not contained in blocks

```

```
>>> pd.__version__
'0.11.1.dev-8a242d2'
```
