isin method buggy on SparseSeries
The isin method doesn't seem to work correctly for SparseSeries objects, but does work on an equivalent dense object, example code below:

``` python
In [203]: pandas.__version__
Out[203]: '0.7.3'

In [204]: sparse_df = DataFrame({'flag': [1., 0., 1.]}).to_sparse(fill_value=0.)

In [205]: sparse_df[sparse_df.flag == 1.]
Out[205]: 
   flag
0     1
2     1

In [206]: sparse_df[sparse_df.flag.isin([1.])]
Out[206]: 
   flag
0     1
1     0

In [207]: dense_df = DataFrame({'flag': [1., 0., 1.]})

In [208]: dense_df[dense_df.flag == 1.]
Out[208]: 
   flag
0     1
2     1

In [209]: dense_df[dense_df.flag.isin([1.])]
Out[209]: 
   flag
0     1
2     1
```
