BUG: Boolean indexed assignment
Take the simple example below:

```
s = pd.Series([1.0, 2.0], dtype=float)
s[[True, False]] = np.array([2.0])
```

The expectation would be that the first item in the series will be set to 2. Unfortunately due to a bug in broadcasting, pandas will set the first item in the series to 2*len(s). The culprit appears to be the below code (series.py:757 - where() method definition):

```
if len(other) == 1:
    other = np.array(other[0]*len(ser))
```

`other` in this case is the length 1 array we need to broadcast. I believe the code made the assumption that other will be perhaps a list of lists? As it stands, this code will take the single element out of the array, multiply it by the length of the series and then carry out the update.

I propose that we fix this by using np.repeat which does the correct thing (pending testing).
