BUG: One item lists are assigned into dataframes as single items
#### Code Sample, a copy-pastable example if possible
```python
df = pd.DataFrame(data={'A':[1, 2, 3, 4], 'B':[5, 6, 7, 8], 'C':[8, 9, 10, 13]})
df['new_col'] = np.nan
df.loc[[0,1], 'new_col'] = [['one_item'], ['one_item']]
df.loc[[2,3], 'new_col'] = [['one_item'], ['two', 'items']]
print(df)
#     A  B   C         new_col
# 0  1  5   8           one_item
# 1  2  6   9           one_item
# 2  3  7  10          [one_item]
# 3  4  8  13          [two, items]

```
#### Problem description

I would like to create a new column and load the values into it gradually, however in the first assignment to the 'new_col' as the list has only one item they are assigned as items and not as lists. 

#### Expected Output

```python
#    A  B   C         new_col
# 0  1  5   8          [one_item]
# 1  2  6   9          [one_item]
# 2  3  7  10          [one_item]
# 3  4  8  13          [two, items]
```
#### Output of ``pd.show_versions()``
<details>

INSTALLED_VERSIONS
-------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.20.2
pytest: None
pip: 9.0.1
setuptools: 27.2.0
Cython: None
numpy: 1.13.0
scipy: 0.19.1
xarray: None
IPython: 5.4.1
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
