DataFrame.plot(): changing position of legend with secondary_y removes legend entries
#### Code Sample

```python
df = pd.DataFrame([[1, 1], [2, 4], [3, 7]], index=[1, 2, 3],
                  columns=['col a', 'col b'])
df.plot(kind='bar', secondary_y=['col b']).legend(bbox_to_anchor=(1.5, 1))
```
#### Problem description

The code above produces a barplot with secondary y axis for 'col b'.
The last command (.legend(...)) tells it to move the legend around, which works but produces a new legend that only includes 'col a'.

#### Expected Output

I believe it should include both columns in the legend, like it does when the .legend part is omitted.
I have also tried to move the legend around with other methods, but with no success.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 8.1.2
setuptools: 35.0.2
Cython: 0.24.1
numpy: 1.12.1
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 6.0.0
sphinx: 1.4.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: 0.7.9.None
psycopg2: None
jinja2: 2.8
boto: 2.42.0
pandas_datareader: None
</details>
