TST: restore test of invalid caching string

Restore test of not-valid string specifycing caching to `get_data` and
`get_fdata`.

Author: matthew-brett

nibabel/tests/test_image_api.py

@@ -286,6 +286,9 @@ def validate_data_interface(self, imaker, params):
                     # cache state.
                     img.uncache()
                     assert_true(img.in_memory)
+                # Values to get_data caching parameter must be 'fill' or
+                # 'unchanged'
+                assert_raises(ValueError, img.get_data, caching='something')
             # Data shape is same as image shape
             assert_equal(img.shape, method().shape)
             # Values to get_data caching parameter must be 'fill' or