Ending blank lines always reformatted no matter --lines args

Clarification of issue #746. Any trailing blank lines are deleted. This should not be the case when they aren't in any of the ranges mentioned by the '--lines' argument.

Running `yapf -l 1-2` on:
```python
a = 4
b = 5

```
deletes lines 3-4 and gives
```python
a=4
b=5
```
instead of the expected original file.