BUG: Not properly checking if post_processing function is callable or not in _get_cythonized_result() function in class GroupBy
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.



#### Problem description
While looking at the code, I found this line which I think is a bug. In the current master branch, pandas/core/groupby/groupby.py file, in the _get_cythonized_result() function :

```
        if post_processing:
            if not callable(pre_processing):
                raise ValueError("'post_processing' must be a callable!")
```

While checking if post_processing is callable or not, we are actually passing pre_processing object instead of post_processing. This looks like a bug.

#### Expected Code
```
        if post_processing:
            if not callable(post_processing):
                raise ValueError("'post_processing' must be a callable!")
```
