masked values not converted to nan when slicing

[gerritholl]

Closely related to #4592, when a slice containing masked values is assigned to an unmasked float array, the underlying data are copied, but masked elements are not converted to nan:

In [289]: A = arange(5, dtype="f2")

In [290]: fm = ma.masked_array([1.0, 2.0, 3.0, 4.0], [True, False, True, False])

In [291]: A[0] = fm[0]
/dev/shm/gerrit/venv/stable-3.5/bin/ipython3:1: UserWarning: Warning: converting a masked element to nan.
  #!/dev/shm/gerrit/venv/stable-3.5/bin/python3.5

In [292]: print(A)
[ nan   1.   2.   3.   4.]

In [293]: A[:4] = fm[:4]

In [294]: print(A)
[ 1.  2.  3.  4.  4.]