BUG: df.plot - Changing the legend does not work for shared_y
#### Code Sample, a copy-pastable example if possible

```python
# Your code here

def plot_outlier_stats(df_joined_spatial,include_std=False):
    columns = ["+ Mean","+  Mean (%)","- Mean",'-  Mean (%)']
    if include_std:
        columns += ['+ Std','- Std']
    ax = df_joined_spatial[columns].plot(kind="bar",secondary_y=["+  Mean (%)",'-  Mean (%)'])
    ax.legend(loc='upper center',)
    return ax

vals = array([[4.86153846e+01, 1.18689904e+00, 4.35006631e+00, 2.51538462e+01,
        6.14107572e-01, 7.54813613e+00],
       [6.92307692e+00, 1.69020433e-01, 7.78311782e+00, 2.30769231e+00,
        5.63401442e-02, 2.59437261e+00],
       [6.15384615e-01, 1.20192308e-01, 1.50213523e+00, 1.53846154e-01,
        3.00480769e-02, 3.75533808e-01],
       [0.00000000e+00, 0.00000000e+00, 0.00000000e+00, 2.07692308e+00,
        4.05649038e-01, 2.77350098e-01],
       [6.46153846e+00, 1.26201923e+00, 2.14535874e+00, 2.15384615e+00,
        4.20673077e-01, 8.98717034e-01]])

df_test = pd.DataFrame(vals)
df_test.columns = ['+ Mean', '+  Mean (%)', '+ Std', '- Mean', '-  Mean (%)', '- Std']
plot_outlier_stats(df_test)
```
#### Problem description

The resulting legend should include columns from both axes(["+ Mean","+  Mean (%)","- Mean",'-  Mean (%)']); however,  it only includes the texts of the left axis.

#### Expected Output

legend with ["+ Mean","+  Mean (%)","- Mean",'-  Mean (%)'] what is return is only ["+ Mean","- Mean"]. 
Also,

` legend = ax.__dict__['legend_'].__dict__["texts"]` returns all texts, but for `h1, l1 = ax.get_legend_handles_labels()` l1 = ['+ Mean', '- Mean'] it should be all
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-1005-azure
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.0.3
pip: 9.0.1
setuptools: 38.5.1
Cython: 0.26.1
numpy: 1.14.1
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.5.4
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.1
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.2.1
pymysql: None
psycopg2: 2.7.3.1 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: 0.1.3
pandas_gbq: None
pandas_datareader: 0.2.1
</details>
