BUG: Don't include array-like repr in plot titles and axis label when using array inputs in dataframe.boxplot() 
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.random((200,1)),  columns = ['value'])
df['date'] = pd.date_range('2000-1-1', periods=200, freq='D')

df.boxplot(column='value',by=df['date'].dt.month)
```
#### Problem description
The result shown below explains a lot so I am not going to elaborate much.

As far as I tested, this only happens if datetime function is used for grouping values in dataframe.boxplot function, in terms of by=df['datetime_column'].dt.function or other datetime functions.




#### Expected Output
![untitled](https://user-images.githubusercontent.com/43888085/46546309-42ed6200-c8fb-11e8-9602-84e195818635.png)
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.4
pytest: None
pip: 10.0.1
setuptools: 40.2.0
Cython: None
numpy: 1.15.2
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.5.0
sphinx: 1.8.1
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
