pd.Series.asof performance regression: 14x slower
#### pd.Series.asof takes 14x longer in 0.19.0 than 0.18.1

``` python
In[14]: pd.__version__
Out[14]: '0.18.1'
In[15]: s = pd.Series([1, 1, 1, np.nan, np.nan])
In[16]: %timeit s.asof(4)
The slowest run took 8.25 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 8.96 µs per loop

In[8]: pd.__version__
Out[8]: '0.19.0'
In[9]: s = pd.Series([1, 1, 1, np.nan, np.nan])
In[10]: %timeit s.asof(4)
10000 loops, best of 3: 125 µs per loop
```
