BUG: INCREF before DECREF when copying object arrays.

Only the change in dtype_transfer actually seems to be
related to the simple np.copyto(a, a) bug, when a is a scalar
object array.
It seemed however safer to me, to generally turn around the
logic, even if it may not be necessary and in one place
requires an extra temporary variable.

Closes gh-3787
Merge pull request #3788 from seberg/object-copy-refs

BUG: INCREF before DECREF when copying object arrays.