tm.makeObjectSeries returns a Series with dtype="datetime64[ns]"
```python-traceback
>>> import pandas.util.testing as tm
>>> tm.makeObjectSeries()
...
t1Q4jcW3nW   2000-02-09
ElZCr3WTz2   2000-02-10
p43Nqg7u4D   2000-02-11
dtype: datetime64[ns]
```

@jbrockmendel and I were both surprised that calling `makeObjectSeries` returns a Series with a datetime dtype. I think it would be more logical for this function to actually return an object dtype Series