Fix astype from boolean

[takagi]

Fix #4354.

The result of astype from a ndarray whose dtype is boolean to another dtype is expected to be zero or one. However, its underlying representation is not necessarily zero or one (i.g. a view). And in such a case, astype returns a ndarray with values other than zero or one. This PR fixes astype from boolean ndarray to return the proper value.

[toslunar]

Does the change also fix automatic conversion from boolean?

>>> for xp in (numpy, cupy): xp.arange(3, dtype=xp.int8).view(bool) + 4.5
...
array([4.5, 5.5, 5.5])
array([4.5, 5.5, 6.5])

[takagi]

I got the same result with this change, so it seems to be another issue.

[toslunar]

LGTM because fixing the astype issue can have higher priority than other type conversions.

[toslunar]

Jenkins, test this please.

[chainer-ci]

Jenkins CI test (for commit c576e99, target branch master) succeeded!