Can't put date in Series if index is a string longer than 1 character

[emberlightstudios]

Code Sample

>>> import pandas
>>> x = pandas.Series([1,2,3],  index=['Date','b','other'])
>>> x
Date     1
b        2
other    3
dtype: int64
>>> from datetime import date
>>> x.Date = date.today()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python37\lib\site-packages\pandas\core\generic.py", line 4405, in __setattr__
    self[name] = value
  File "C:\Python37\lib\site-packages\pandas\core\series.py", line 939, in __setitem__
    setitem(key, value)
  File "C:\Python37\lib\site-packages\pandas\core\series.py", line 935, in setitem
    self._set_with(key, value)
  File "C:\Python37\lib\site-packages\pandas\core\series.py", line 983, in _set_with
    self._set_labels(key, value)
  File "C:\Python37\lib\site-packages\pandas\core\series.py", line 993, in _set_labels
    raise ValueError('%s not contained in the index' % str(key[mask]))
ValueError: ['D' 'a' 't' 'e'] not contained in the index
>>> x.b = date.today()
>>> x.b
datetime.date(2018, 11, 1)
>>> x
Date              1
b        2018-11-01
other             3
dtype: object
>>>

Problem description

I cannot put a date object in a Series if the index is a string with len > 1.
It works if it's only a single character. Other types seem to work. I've only seen the problem with dates.

[mroeschke]

This looks similar to #12862, and I can reproduce this in master as well.