consider case when all axes chosen

Author: rcomer

lib/iris/coords.py

@@ -2233,8 +2233,9 @@ def collapsed(self, dims_to_collapse=None):
             # Collapse the coordinate by serializing the points and
             # bounds as strings.
             def serialize(x, axis):
-                if axis is None:
+                if axis is None or len(axis) == x.ndim:
                     return "|".join(str(i) for i in x.flatten())
+
                 # np.apply_along_axis does not work with str.join, so we
                 # need to loop through the array directly. First move (possibly
                 # multiple) axis of interest to trailing dim(s), then make a 2D

lib/iris/tests/unit/coords/test_Coord.py

@@ -583,7 +583,7 @@ def test_string_masked(self):
         points = ma.array(["foo", "bar", "bing"], mask=[0, 1, 0], dtype=str)
         coord = AuxCoord(points)
 
-        collapsed_coord = coord.collapsed()
+        collapsed_coord = coord.collapsed(0)
 
         expected = "foo|--|bing"
         self.assertEqual(collapsed_coord.points, expected)