IntervalIndex is_monotonic_decreasing is incorrect
#### Code Sample, a copy-pastable example if possible

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

However, `is_non_overlapping_monotonic` does correctly return `True`:
```python
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()``

<details>

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

</details>
