change test from `type(a) is not mu.ndarray` to
`not isinstance(a, mu.ndarray)`

Because every sub-class of ndarray is not guaranteed to implement
`keepdims` as a kwarg, when wrapping these methods care must be taken.

The previous behavior was to silently eat the kwarg when dealing with
a sub-class of ndarray.

Now, if `keepdims=np._NoValue` (the new default) it is not passed
through to the underlying function call (so the default value of
`keepdims` is now controlled by the sub-class).  If `keepdims` is not
`np._NoValue` then it is passed through and will raise an exception if
the sub-class does not support the kwarg.

A special case in nanvar was required to deal with `matrix` that previously
relied on `fromnumeric` silently dropping `keepdims`.