BUG: IntervalIndex.insert does not work for inserting NaN
#### Code Sample, a copy-pastable example if possible
```python
In [3]: ivs = pd.interval_range(0, 4)
   ...: ivs
   ...:
Out[3]:
IntervalIndex([(0, 1], (1, 2], (2, 3], (3, 4]]
              closed='right',
              dtype='interval[int64]')

In [4]: ivs.insert(2, np.nan)
---------------------------------------------------------------------------
ValueError: can only insert Interval objects into an IntervalIndex
```
#### Problem description
`IntervalIndex.insert` fails to insert `NaN`.

Note that `NaN` should be valid to insert, as you can construct an `IntervalIndex` with `NaN`:
```python
In [5]: pd.IntervalIndex([pd.Interval(0, 1), np.nan, pd.Interval(2, 3)])
Out[5]:
IntervalIndex([(0.0, 1.0], nan, (2.0, 3.0]]
              closed='right',
              dtype='interval[float64]')
```

Additionally, other types of index allow for `NaN` to be inserted:
```python
In [6]: pd.Index(list('abc')).insert(2, np.nan)
Out[6]: Index(['a', 'b', nan, 'c'], dtype='object')

In [7]: pd.period_range('2017Q1', periods=4, freq='Q').insert(2, np.nan)
Out[7]: PeriodIndex(['2017Q1', '2017Q2', 'NaT', '2017Q3', '2017Q4'], dtype='period[Q-DEC]', freq='Q-DEC')
```
#### Expected Output
`NaN` to be inserted at the specified location:
```python
IntervalIndex([(0.0, 1.0], (1.0, 2.0], nan, (2.0, 3.0], (3.0, 4.0]]
              closed='right',
              dtype='interval[float64]')
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.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: 0.21.0
pytest: 3.1.2
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.26
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.4
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
