Where method on DataFrames can't seem to take series as conditions
I want to use `where` to set values across a DataFrame where a Series meets a condition.

For example:

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

But this 

``` python
df.where(df['a']==2,0)
```

returns an unedited DataFrame. 
Whereas this returns just the middle row, as you'd expect:

``` python
df[df['a']==2]
```

Supplying `axis=0` or `axis=1` doesn't help. Am I doing something wrong? Or this unintended behavior?

> Versions
> python: 2.7.6.final.0
> pandas: 0.14.1
