Timedelta interpreted as int upon first insertion into Series

[kernc]

Code Sample, a copy-pastable example if possible

>>> s = pd.Series(['x'])
>>> s['td'] = pd.Timedelta('9 days')
>>> s
0                   x
td    777600000000000
dtype: object

# However, *if the key already exists*, it works as expected:
>>> s['td_2'] = 0
>>> s['td_2'] = pd.Timedelta('9 days')
>>> s
0                    x
td     777600000000000
td_2   9 days 00:00:00
dtype: object

Problem description

Timedeltas should remain Timedeltas when inserted into object Series, particularly because they are nice to work with. 😅

Expected Output

>>> s = pd.Series(['x'])
>>> s['td'] = pd.Timedelta('9 days')
>>> s
0                    x
td     9 days 00:00:00
dtype: object

Output of pd.show_versions()

pandas 0.23.3

[mroeschke]

xref #13829