DataFrame resample and Grouper ,Why are the results different?
#### Code Sample, a copy-pastable example if possible

data:
```python
idx = pd.date_range('2017-1-1','2018-12-31',freq='M')
df = pd.DataFrame(np.arange(len(idx)),idx)
```

resample:
```python
df.resample('3m',kind='period').sum()
```

use grouper:
```python
df.groupby(pd.Grouper(level=0,freq='3m',kind='period')).sum()
```

#### Problem description
Why are the results different?

#### Expected Output
I hope that the result of using the grouper is the same as the resample method.

thank you!!


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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.0
numpy            : 1.16.2
pytz             : 2018.9
dateutil         : 2.8.0
pip              : 19.0.3
setuptools       : 40.8.0
Cython           : 0.29.6
pytest           : 4.3.1
hypothesis       : None
sphinx           : 1.8.5
blosc            : None
feather          : 0.4.0
xlsxwriter       : 1.1.5
lxml.etree       : 4.3.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 7.4.0
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.2
matplotlib       : 3.0.3
numexpr          : 2.6.9
odfpy            : None
openpyxl         : 2.6.1
pandas_gbq       : None
pyarrow          : 0.13.0
pytables         : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : 1.3.1
tables           : 3.5.1
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.1.5

</details>
