Multiple aggregations with DatetimeIndexResamplerGroupby raises
On master, doing a `groupby().resample.agg` with multiple aggfuncs fails.

```python
In [13]: df = pd.DataFrame({"A": pd.to_datetime(['2015', '2017']), "B": [1, 1]})

In [14]: df
Out[14]:
           A  B
0 2015-01-01  1
1 2017-01-01  1

In [15]: df.set_index("A").groupby([0, 0]).resample("AS")
Out[15]: DatetimeIndexResamplerGroupby [freq=<YearBegin: month=1>, axis=0, closed=left, label=left, convention=e, base=0]

In [16]: df.set_index("A").groupby([0, 0]).resample("AS").agg(['sum', 'count'])
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-16-5f1c18a8d4ac> in <module>()
----> 1 df.set_index("A").groupby([0, 0]).resample("AS").agg(['sum', 'count'])

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/resample.py in aggregate(self, arg, *args, **kwargs)
    339
    340         self._set_binner()
--> 341         result, how = self._aggregate(arg, *args, **kwargs)
    342         if result is None:
    343             result = self._groupby_and_aggregate(arg,

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _aggregate(self, arg, *args, **kwargs)
    538             return self._aggregate_multiple_funcs(arg,
    539                                                   _level=_level,
--> 540                                                   _axis=_axis), None
    541         else:
    542             result = None

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _aggregate_multiple_funcs(self, arg, _level, _axis)
    583                 try:
    584                     colg = self._gotitem(col, ndim=1, subset=obj[col])
--> 585                     results.append(colg.aggregate(arg))
    586                     keys.append(col)
    587                 except (TypeError, DataError):

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/resample.py in aggregate(self, arg, *args, **kwargs)
    339
    340         self._set_binner()
--> 341         result, how = self._aggregate(arg, *args, **kwargs)
    342         if result is None:
    343             result = self._groupby_and_aggregate(arg,

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _aggregate(self, arg, *args, **kwargs)
    538             return self._aggregate_multiple_funcs(arg,
    539                                                   _level=_level,
--> 540                                                   _axis=_axis), None
    541         else:
    542             result = None

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _aggregate_multiple_funcs(self, arg, _level, _axis)
    582             for col in obj:
    583                 try:
--> 584                     colg = self._gotitem(col, ndim=1, subset=obj[col])
    585                     results.append(colg.aggregate(arg))
    586                     keys.append(col)

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in _gotitem(self, key, ndim, subset)
    675                        for attr in self._attributes])
    676         self = self.__class__(subset,
--> 677                               groupby=self._groupby[key],
    678                               parent=self,
    679                               **kwargs)

~/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/base.py in __getitem__(self, key)
    241         if self._selection is not None:
    242             raise Exception('Column(s) {selection} already selected'
--> 243                             .format(selection=self._selection))
    244
    245         if isinstance(key, (list, tuple, ABCSeries, ABCIndexClass,

Exception: Column(s) B already selected
```

A single aggfunc is OK.