[BUG] Index.is_monotonic_* methods not factoring nan values

[galipremsagar]

Describe the bug
When nan values are present, Index.is_montonic_* methods seem to be returning incorrect results:

Steps/Code to reproduce bug

In [1]: import pandas as pd

In [2]: import numpy as np

In [3]: idx = pd.Index([1.0, 2.0, np.nan])

In [4]: idx
Out[4]: Index([1.0, 2.0, nan], dtype='float64')

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

In [6]: import cudf

In [7]: gidx = cudf.from_pandas(idx, nan_as_null=False)

In [8]: gidx
Out[8]: Index([1.0, 2.0, nan], dtype='float64')

In [9]: gidx.is_monotonic_increasing
Out[9]: True

Expected behavior
Match pandas.