Series.sort_index(inplace=True) always returns a new Series
Series.index_sort() _always_ returns a new Series, ignoring the `inplace` keyword argument. This is in contrast to DataFrame's sort_index() which works as expected.

``` python
vals = range(10, 0, -1)
x = pd.Series(vals)

# Returns a new Series with the sorted index instead of
# changing x in place
y = x.sort_index(ascending=False, inplace=True)

# Not the same
(x != y).all()

# sort_index() works as expected for DataFrames
xx = pd.DataFrame({'col': x})
assert xx.sort_index(ascending=False, inplace=True) is None
```

show_versions() output:
## INSTALLED VERSIONS

commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.0
nose: 1.3.7
pip: 7.1.2
setuptools: 18.4
Cython: None
numpy: 1.10.1
scipy: 0.16.0
statsmodels: 0.6.1
IPython: 4.0.0
sphinx: None
patsy: 0.4.0
dateutil: 2.4.1
pytz: 2015.6
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.4.4
matplotlib: 1.4.3
openpyxl: 2.2.0-b1
xlrd: 0.9.3
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.3
pymysql: 0.6.6.None
psycopg2: None
