BUG: query modifies the frame when you compare with `=`

I messed up and used `=` instead of `==` in a `query`.

``` python
df = pd.DataFrame({'a': [1, 2, 3], 'b': ['a', 'b', 'c']})
df.query('a=1')
```

That raises a ValueError. But `df` was modified.

``` python
In [15]: df
Out[15]:
   a  b
0  1  a
1  1  b
2  1  c
```

versions:

```
pandas: 0.15.0-6-g403f38d
bottleneck: None
tables: None
numexpr: 2.3.1
```

Can't look right now.
