Pandas ignores index when assigning different dtypes.
Consider the following example:

``` python
import pandas

foo = pandas.Series( [ 0, 1, 2, 0 ] )

w   = foo > 0
bar = foo[ w ].map( str )
print bar

foo[w] = bar
print foo

foo[w] = bar
print foo
```

gives the following output

```
1    1
2    2
dtype: object

0      0
1    NaN
2      1
3      0
dtype: object

0    0
1    1
2    2
3    0
dtype: object
```

It looks as if in the first assignemend `foo[w] = bar` the matching index of both sides is ignored.

Is this a bug or documented behavior?
