GroupBy.apply repeats the first group N times when slicing and sorting in the applied function
Hi,

When using GroupBy.apply on function that applies pandas.DataFrame.sort_values() **with inplace=True** and also **data-frame slicing**, the first group is evaluated N times (N is the original number of the groups).

See the following code and output:

```python
>>> import numpy as np
>>> import pandas as pd
>>> 
>>> A = np.repeat('a', 8)
>>> B = np.repeat([1, 2, 3, 4], 2)
>>> data = pd.DataFrame.from_dict({'A': A, 'B': B})
>>> print(data)
   A  B
0  a  1
1  a  1
2  a  2
3  a  2
4  a  3
5  a  3
6  a  4
7  a  4
>>> 
>>> def func(group_data, sort, slice):
...     if sort:
...         group_data.sort_values('A', inplace=True)
...     if slice:
...         group_data = group_data[:]
...     return group_data
... 
>>> B_groups = data.groupby('B')
>>> out_sort_slice = B_groups.apply(lambda x: func(x, sort=True, slice=True))
>>> out_sort_no_slice = B_groups.apply(lambda x: func(x, sort=True, slice=False))
>>> out_no_sort_slice = B_groups.apply(lambda x: func(x, sort=False, slice=True))
>>> out_no_sort_no_slice = B_groups.apply(lambda x: func(x, sort=False, slice=False))
>>> 
>>> print(out_sort_slice)
     A  B
B        
1 0  a  1
  1  a  1
2 0  a  1
  1  a  1
3 0  a  1
  1  a  1
4 0  a  1
  1  a  1
>>> print(out_sort_no_slice)
   A  B
0  a  1
1  a  1
2  a  2
3  a  2
4  a  3
5  a  3
6  a  4
7  a  4
>>> print(out_no_sort_slice)
     A  B
B        
1 0  a  1
  1  a  1
2 2  a  2
  3  a  2
3 4  a  3
  5  a  3
4 6  a  4
  7  a  4
>>> print(out_no_sort_no_slice)
   A  B
0  a  1
1  a  1
2  a  2
3  a  2
4  a  3
5  a  3
6  a  4
7  a  4
```

As you can see,  out_sort_no_slice, out_no_sort_slice and out_no_sort_no_slice get the expected outputs, while the output of out_sort_slice contains the first group 4 times. while debugging, you can see the first group evaluated 4 times inside func.
Notice that the data is already sorted by field 'A' and the slicing part keeps all the data in the data-frame.

I'm using Anaconda Python 3.7 with Pandas 0.24.2 on Mac.

Thanks!