Timedelta interpreted as int upon first insertion into Series
#### Code Sample, a copy-pastable example if possible

```python
>>> 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. :sweat_smile: 

#### Expected Output

```python
>>> 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()``

<details>

pandas 0.23.3

</details>
