If the items in self.prev contain trailing newlines, they should expect to get data that comes from that:

```python
>>> import dpark
>>> data = ['%d\n' % i for i in range(4)]
>>> data
['0\n', '1\n', '2\n', '3\n']
>>> list(dpark.makeRDD(data).pipe('cat').collect())
['0', '', '1', '', '2', '', '3', '']
```

If there are newlines in my data, I would expect that they would interact with my command as specified. The newlines that I'm appending to the data are only required as that's the mechanism by which I send data through the pipe to the command.  It's not data.