IntervalIndex is_monotonic_decreasing is incorrect

[jschendel]

Code Sample, a copy-pastable example if possible

In [2]: idx = pd.IntervalIndex.from_tuples([(2, 3), (1, 2), (0, 1)], closed='neither')

In [3]: idx
Out[3]:
IntervalIndex([(2, 3), (1, 2), (0, 1)]
              closed='neither',
              dtype='interval[int64]')

In [4]: idx.is_monotonic_decreasing
Out[4]: False

This also incorrectly returns False for is_monotonic:

In [5]: idx.is_monotonic
Out[5]: False

However, is_non_overlapping_monotonic does correctly return True:

In [6]: idx.is_non_overlapping_monotonic
Out[6]: True

Problem description

The intervals are monotonic decreasing but the is_monotonic_decreasing and is_monotonic attributes are returning False.

Expected Output

I'd expect both is_monotonic_decreasing and is_monotonic to return True.

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.1
pytest: 3.0.7
pip: 9.0.1
setuptools: 34.4.1
Cython: 0.25.2
numpy: 1.13.0rc2
scipy: 0.18.1
xarray: 0.9.5
IPython: 5.3.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: 1.4.4
bottleneck: 1.2.0
tables: 3.4.2
numexpr: 2.6.1
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.0
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.7.3
bs4: None
html5lib: None
sqlalchemy: 1.1.10
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: 0.1.6
pandas_datareader: 0.4.0

[jschendel]

Looks like this is a dupe of #16554. I initially thought they were distinct issues, as a MultiIndex isn't explicitly being used, but it looks like the IntervalIndex version of is_monotonic_decreasing just borrows the MultiIndex version though _multiindex.