inplace argument accepted by DataFrame.rename_axis(newname,axis=1, inplace=True) but is ignored
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({'Column 1'     : [1., 2., 3., 4.],
        'Index Title'  : ["Apples", "Oranges", "Puppies", "Ducks"]})
print df.axes

d2 = df.rename_axis('mycols', axis=1, inplace = True)
print d2
print df
```
#### Problem description
When trying to change the name of an axis using df.rename_axis, the inplace=True option should work but does nothing. The original DataFrame is unchanged.  I get:

```
[RangeIndex(start=0, stop=4, step=1), Index([u'Column 1', u'Index Title'], dtype='object')]
mycols  Column 1 Index Title
0            1.0      Apples
1            2.0     Oranges
2            3.0     Puppies
3            4.0       Ducks
   Column 1 Index Title
0       1.0      Apples
1       2.0     Oranges
2       3.0     Puppies
3       4.0       Ducks
```

#### Expected Output
`d2` and `df` should end up with the same values in my example.

#### Output of ``pd.show_versions()``
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.14.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-22-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.20.3
pytest: None
pip: 9.0.1
setuptools: 36.4.0
Cython: 0.24
numpy: 1.13.3
scipy: 0.17.0
xarray: None
IPython: 5.4.1
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.14
pymysql: None
psycopg2: 2.7.3 (dt dec mx pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>

  