Type conversions are skipped in 'to_dict' on single column dataframes
Code to reproduce the error:

```python
import pandas as pd
from datetime import datetime

dfs = {
    'full_df': pd.DataFrame([
        {'int': 1, 'date': datetime.now(), 'str': 'foo', 'float': 1.0, 'bool': True},
    ]),
    'int_df': pd.DataFrame([
        {'int': 1},
    ]),
    'date_df': pd.DataFrame([
        {'date': datetime.now()},
    ]),
    'str_df': pd.DataFrame([
        {'str': 'foo'},
    ]),
    'float_df': pd.DataFrame([
        {'float': 1.0},
    ]),
    'bool_df': pd.DataFrame([
        {'bool': True},
    ])
}

for name, frame in dfs.items():
    print('Types in ' + name)
    for k, v in frame.to_dict('records')[0].items():
        print(type(v))
```
Output:
```python
Types in full_df
<class 'bool'>
<class 'pandas._libs.tslibs.timestamps.Timestamp'>
<class 'float'>
<class 'int'>
<class 'str'>
Types in int_df
<class 'numpy.int64'>
Types in date_df
<class 'pandas._libs.tslibs.timestamps.Timestamp'>
Types in str_df
<class 'str'>
Types in float_df
<class 'numpy.float64'>
Types in bool_df
<class 'numpy.bool_'>
```
#### Problem description

One would expect that the `to_dict()` function returns python native types, or at least does the same to the same type of columns, however it behaves differently as shown above. It seems that type conversion is not invoked when a single column is present in the dataframe.

#### Expected Output

Python native types where it is possible for `int`, `float`, `bool` and `str` types, and if possible, a python `datetime` object instead of `pandas.Timestamp`

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.0
pytest: None
pip: 9.0.1
setuptools: 38.2.4
Cython: None
numpy: 1.14.3
scipy: None
pyarrow: None
xarray: None
IPython: 6.3.1
sphinx: 1.7.4
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.2
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
