unwrap_phase returns masked array when image is masked

With commit e3b84ed the unwrap_phase function would always return an ndarray
even when image was masked.  This restores the pre-e3b84ed behavior of
returning a masked array when the image is masked.

skimage/restoration/tests/test_unwrap.py

@@ -131,6 +131,7 @@ def test_mask():
         # The end of the unwrapped array should have value equal to the
         # endpoint of the unmasked ramp
         assert_array_almost_equal_nulp(image_unwrapped[:, -1], image[i, -1])
+        assert np.ma.isMaskedArray(image_unwrapped)
 
         # Same tests, but forcing use of the 3D unwrapper by reshaping
         with expected_warnings(['length 1 dimension']):

skimage/restoration/unwrap.py

@@ -89,11 +89,11 @@ def unwrap_phase(image, wrap_around=False, seed=None):
 
     if np.ma.isMaskedArray(image):
         mask = np.require(np.ma.getmaskarray(image), np.uint8, ['C'])
-        image = np.ma.getdata(image)
     else:
         mask = np.zeros_like(image, dtype=np.uint8, order='C')
 
-    image_not_masked = np.asarray(image, dtype=np.double, order='C')
+    image_not_masked = np.asarray(
+        np.ma.getdata(image), dtype=np.double, order='C')
     image_unwrapped = np.empty_like(image, dtype=np.double, order='C')
 
     if image.ndim == 1: