Performance degradation in __setitem__ from 0.12 to 0.15.2
```
In [1]: import pandas as pd

In [2]: print pd.__version__
0.12.0

In [3]: s = pd.Series({'AAPL': 20.})

In [4]: %timeit s['AAPL'] = 25.
1000000 loops, best of 3: 1.29 µs per loop
```

VS

```
In [1]: import pandas as pd

In [2]: print pd.__version__
0.15.2

In [3]: s = pd.Series({'AAPL': 20.})

In [4]: %timeit s['AAPL'] = 25.
100000 loops, best of 3: 6.05 µs per loop
```

Slow down of ~470%. Is this expected? Any way to speed this up?
