BUG: resampling a MultiIndex and specified keys are tuples in .agg
#### Code Sample, a copy-pastable example if possible

You can reproduce the error by pasting this chunk of code, but this error is ONLY with pandas 0.18.0 or higher. This chunk of code worked fine under 0.17.1 and earlier, and is related to the change in how resampling is done.

The error I get is "TypeError: other must be a MultiIndex or a list of tuples"

```
import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.randint(0, 100, (10, 3)), columns=list('ABC'),
                  index=pd.date_range('2016-01-01', freq='1400T', periods=10))
df['D'] = ['x', 'x', 'x', 'x', 'x', 'y', 'y', 'y', 'y', 'y']
df.index.name = "timestamp"
df_pivot = pd.pivot_table(df.reset_index(), values=['A', 'B'], index="timestamp",
                          columns=['C', 'D'])
resample_methods = map({'A': 'sum', 'B': 'mean'}.get, [x[0] for x in df_pivot.columns])
resample_method_tuple_list = zip(df_pivot.columns, resample_methods)
resample_dict = dict(resample_method_tuple_list)
df_pivot.resample('D').agg(resample_dict)
```

Note that this works just fine if I only pivot on one column
#### Expected Output

This should resample the dataframe, without error
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Darwin
OS-release: 15.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 23.0.0
Cython: 0.24
numpy: 1.11.1
scipy: 0.18.0
statsmodels: 0.6.1
xarray: 0.8.0
IPython: 5.0.0
sphinx: 1.4.1
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: 1.1.0
tables: 3.2.3.1
numexpr: 2.6.0
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.2
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: 0.9.2
apiclient: 1.5.1
sqlalchemy: 1.0.13
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: 2.40.0
pandas_datareader: None
