0.23.0 no longer produces TypeError when calling DataFrame.sort_index(level=x) on multiindex of mixed types
#### Code Sample, a copy-pastable example if possible

```python
# Column 'a' has both Timestamps and a string
df = pd.Series([pd.Timestamp('2011/4/9'), pd.Timestamp('2010/4/9'), '2009/4/9'], dtype='object', name='a').to_frame()
df['b'] = [1,2,3]
df['c'] = [2,1,3]
df.set_index('a', inplace=True)
df.set_index('c', append=True, inplace=True)
# No TypeError
df.sort_index(level='a')
# Raises TypeError: Cannot compare type 'Timestamp' with type 'str'
df.reset_index(level='c').sort_index(level='a')

```
#### Problem description

Sorting by mixed types should (and did in 0.22) raise a TypeError. Now it looks like it does raise a TypeError if there is only a single level in the index, but does not if there is another level that you're not sorting on.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.14.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.23.0
pytest: 3.2.3
pip: 10.0.1
setuptools: 36.5.0.post20170921
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.4.1
sphinx: 1.6.4
patsy: None
dateutil: 2.7.2
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: 2.5.1
feather: None
matplotlib: 2.1.1
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: 1.2.7
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: 0.1.2
fastparquet: 0.1.5
pandas_gbq: None
pandas_datareader: None

</details>
