loffset doesn't work in resample if used with agg()
#### Code Sample, a copy-pastable example if possible

```
# Create DF
df = pd.DataFrame(np.random.rand(5,2), columns=list('AB'), index=pd.date_range('2010-01-01 09:00:00', periods=5, freq='s'))

print(df)
                                       A                    B
2010-01-01 09:00:00             0.283113             0.559642
2010-01-01 09:00:01             0.754942             0.621557
2010-01-01 09:00:02             0.102002             0.892100
2010-01-01 09:00:03             0.885400             0.524359
2010-01-01 09:00:04             0.324761             0.706758

#Resample, loffset works fine using mean(), is offset by 2 hours.
print(df.resample('2s', loffset='2h').mean())
                                       A                    B
2010-01-01 11:00:00             0.519028             0.590599
2010-01-01 11:00:02             0.493701             0.708229
2010-01-01 11:00:04             0.324761             0.706758


# Resample with agg(), loffset doesn't work anymore.
print(df.resample('2s', loffset='2h').agg(dict(A='sum', B='mean')))
                                      A                    B
2010-01-01 09:00:00             1.038055             0.590599
2010-01-01 09:00:02             0.987402             0.708229
2010-01-01 09:00:04             0.324761             0.706758

```
#### Expected Output

I'd expect the same as with mean(), this is how resample worked in the past with resample(how=...).

But maybe I also misunderstood something about the change. If so please enlighten me.

Since I need this to keep things going, I use the following workaround for now, please let me know if this is the way to go or if there's a more efficient way:

```
print(df.resample('2s', loffset='2h').agg(dict(A='sum', B='mean')).tshift(2, 'h'))
```
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Darwin
OS-release: 15.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: de_DE.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 20.3
Cython: 0.23.4
numpy: 1.10.4
scipy: 0.17.1
statsmodels: 0.6.1
xarray: None
IPython: 4.2.0
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.5.1
pytz: 2016.2
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.39.0
pandas_datareader: 0.2.1
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