Error with all boolean indices for Series
The following code works fine with pandas 0.14.1

``` python
import pandas as pd
pd.Series([1, 1], index=[True, False]).drop(True)
```

Output:

``` python
False    1
dtype: int64
```

In pandas 0.15rc1, the code blows up and produces the following error:

```
ValueError                                Traceback (most recent call last)
<ipython-input-35-f9a275b6ce2c> in <module>()
----> 1 pd.Series([1, 1], index=[True, False]).drop(True)

/home/d/.virtualenvs/test/local/lib/python2.7/site-packages/pandas-0.15.0rc1-py2.7-linux-x86_64.egg/pandas/core/generic.pyc in drop(self, labels, axis, level, inplace, **kwargs)
   1531         axis, axis_ = self._get_axis(axis), axis
   1532 
-> 1533         if axis.is_unique:
   1534             if level is not None:
   1535                 if not isinstance(axis, MultiIndex):

/home/d/.virtualenvs/test/local/lib/python2.7/site-packages/pandas-0.15.0rc1-py2.7-linux-x86_64.egg/pandas/lib.so in pandas.lib.cache_readonly.__get__ (pandas/lib.c:38943)()

/home/d/.virtualenvs/test/local/lib/python2.7/site-packages/pandas-0.15.0rc1-py2.7-linux-x86_64.egg/pandas/core/index.pyc in is_unique(self)
    582     def is_unique(self):
    583         """ return if the index has unique values """
--> 584         return self._engine.is_unique
    585 
    586     def is_integer(self):

/home/d/.virtualenvs/test/local/lib/python2.7/site-packages/pandas-0.15.0rc1-py2.7-linux-x86_64.egg/pandas/index.so in pandas.index.IndexEngine.is_unique.__get__ (pandas/index.c:4449)()

/home/d/.virtualenvs/test/local/lib/python2.7/site-packages/pandas-0.15.0rc1-py2.7-linux-x86_64.egg/pandas/index.so in pandas.index.IndexEngine._do_unique_check (pandas/index.c:4901)()

/home/d/.virtualenvs/test/local/lib/python2.7/site-packages/pandas-0.15.0rc1-py2.7-linux-x86_64.egg/pandas/index.so in pandas.index.IndexEngine._ensure_mapping_populated (pandas/index.c:5106)()

/home/d/.virtualenvs/test/local/lib/python2.7/site-packages/pandas-0.15.0rc1-py2.7-linux-x86_64.egg/pandas/index.so in pandas.index.IndexEngine.initialize (pandas/index.c:5203)()

/home/d/.virtualenvs/test/local/lib/python2.7/site-packages/pandas-0.15.0rc1-py2.7-linux-x86_64.egg/pandas/hashtable.so in pandas.hashtable.PyObjectHashTable.map_locations (pandas/hashtable.c:12440)()

ValueError: Does not understand character buffer dtype format string ('?')
```

Is this the expected behaviour in the new release?

cheers
