Dok Slicing gives wrong output.

[Harshit Gupta]

Describe the Bug
Dok Slicing gives wrong output for slice indices = (slice(1, None, -1), slice(None, None, 1))

To Reproduce

import numpy as np
import sparse
s_array = sparse.random((1,3), density=1/3, format='dok')
d_array = s_array.todense()
indices = (slice(1, None, -1), slice(None, None, 1))
print((s_array[indices].todense() == d_array[indices]).all())
# False

Expected behavior
It should print True.

System

• OS and version: Pop Os 20.04
• sparse version (0.12)
• NumPy version (1.20.2)
• Numba version (0.53.1)

[Hameer Abbasi]

This was fixed by #501.