Preserve subclass in `abind` Image-method

[aoles]

The object returned by abind should be of the same class as inputs. The following should evaluate to TRUE.

ImageSubclass <- setClass ("ImageSubclass", contains = "Image", slots = c(foo = "character"))
x <- ImageSubclass(Image(), foo="bar")

identical(
  class(combine(x, x)),
  class(abind(x, x))
)

## FALSE