fix keys arg in eval command

Hi Vlad!
I've noticed that ``eval`` command has undesired behavior:
```python
keys = ['key1', 'key2']
args = ['arg1', 'arg2']
yield gen.Task(self.client.eval, script, keys, args)
print(keys)
>>> ['key1', 'key2', 'arg1', 'arg2']
```
since ``keys`` is mutable list, any changes inside ``eval`` command affect outside ``keys`` variable. 
so here patch and test to fix this issue. 
Please review.