BUG: .transform('size') should be the same as .transform(len)
```
import pandas as pd
N = 6
df = pd.DataFrame(data={"size":[0.1]*N,
                             "group":[None]*N}, 
                       index=range(N))
gb = df.groupby(by=['group'])
d2 =  gb.transform('sum')
print d2
```
#### Expected Output

```
# not actually sure what should be returned, perhaps just the original values
Here is what I got
            size
0  2.605041e-316
1  6.899144e-310
2  2.584561e+161
3  1.485397e-313
4  1.916117e+214
5  3.075369e+203


#### output of ``pd.show_versions()``

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 3.2.0-54-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.0
setuptools: 20.7.0
Cython: 0.24
numpy: 1.11.0
scipy: 0.17.0
statsmodels: 0.6.1
xarray: 0.7.2
IPython: 4.2.0
sphinx: 1.3.6
patsy: 0.4.1
dateutil: 2.2
pytz: 2016.4
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.2.0-b1
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.5
lxml: 3.6.0
bs4: 4.4.1
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.39.0
pandas_datareader: None
```
