Shortcut functions in transform are not grouped
```ipython
$ ipython3
Python 3.6.3 (default, Oct  3 2017, 21:45:48) 
Type 'copyright', 'credits' or 'license' for more information
IPython 6.2.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import pandas as pd

In [2]: df = pd.DataFrame({'A': [1, 1, 2], 'B': [1, 2, 3]})

In [3]: df.groupby('A').transform('rank')
Out[3]: 
   B
0  1
1  1
2  2

In [4]: df.groupby('A').transform(lambda x: x.rank())
Out[4]: 
     B
0  1.0
1  2.0
2  1.0

```
#### Problem description
It seems like the functions provided through shorthand strings, such as 'rank', do not obey the groupings of data. For example, one would expect Out[3] and Out[4] to be the same in the code above. Instead, it seems like using `.transform('rank')` doesn't actually obey the grouping and just ranks independently. I've reproduced this with larger dataframes as well.