BUG Ensure masked object arrays can always return single items.

For a masked array holding objects that themselves are arrays, when selecting a
single item, it is treated as if it were a slice of the array and an attempt is
made to set its mask.  This was always a bug, but it become visible with a
recent change to `MaskedArray.__getitem__` (gh-4586) where it is attempted to
change the shape of the mask.  With this PR, this case gets special  treatment
in that the object is made into a Masked Array with a full set mask.
(A previous attempt to do the perhaps more logical think and just return
`masked` caused quite a few errors in astropy.io.votable; it seemed it was
not worth breaking backwards compatibility that much).

Test case that now works but used to fail:
```
mx1 = MaskedArray([1.], mask=[True])
mx2 = MaskedArray([1., 2.])
mx = MaskedArray([mx1, mx2], mask=[False, True])
mx[0]
```

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