Setting with enlargement fails in presence of MultiIndex
```
In [1]:

import pandas as pd
import numpy as np

print pd.__version__

a = np.linspace(0, 10, 11)
idx = [('test1', i) for i in range(5)]
idx.extend([('test2', i) for i in range(6)])

idx = pd.MultiIndex.from_tuples(idx)
0.15.2-80-ge69deae
In [2]:

a = pd.Series(a, idx)
a
Out[2]:
test1  0     0
       1     1
       2     2
       3     3
       4     4
test2  0     5
       1     6
       2     7
       3     8
       4     9
       5    10
dtype: float64
In [3]:

a.loc[('test1', 2)] = 13
a
Out[3]:
test1  0     0
       1     1
       2    13
       3     3
       4     4
test2  0     5
       1     6
       2     7
       3     8
       4     9
       5    10
dtype: float64
In [4]:

a.loc[('test', 17)] = 13
---------------------------------------------------------------------------
IndexingError                             Traceback (most recent call last)
<ipython-input-4-157acf13152f> in <module>()
----> 1 a.loc[('test', 17)] = 13

/home/federico/Python_Libraries/pandas/pandas/core/indexing.pyc in __setitem__(self, key, value)
    114                 if len(key) > self.ndim:
    115                     raise IndexingError('only tuples of length <= %d supported' %
--> 116                                         self.ndim)
    117                 indexer = self._convert_tuple(key, is_setter=True)
    118             else:

IndexingError: only tuples of length <= 1 supported

In [5]:

a.ix[('test', 17)] = 13
---------------------------------------------------------------------------
IndexingError                             Traceback (most recent call last)
<ipython-input-5-89467846f8af> in <module>()
----> 1 a.ix[('test', 17)] = 13

/home/federico/Python_Libraries/pandas/pandas/core/indexing.pyc in __setitem__(self, key, value)
    114                 if len(key) > self.ndim:
    115                     raise IndexingError('only tuples of length <= %d supported' %
--> 116                                         self.ndim)
    117                 indexer = self._convert_tuple(key, is_setter=True)
    118             else:

IndexingError: only tuples of length <= 1 supported
```

Somehow - the enlarging fails.  Any clue how to get it to work?  I have also noticed that the 'fast' indexing methods, iat and at fail with DataFrames that are multi indexed.
