'base' argument when resampling has no effect
I seem to encounter the similar issue with #10530 which is marked closed.


```
import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.normal(size=(3,4)))
df.index = [pd.Timestamp('2018-02-07'), pd.Timestamp('2018-06-22'), pd.Timestamp('2018-09-17')]
df.resample('6M',base=6).min()
```


the `base` parameter can be anything that doesn't affect the result or generate error (version 0.23.4)