Array field values should be preserved (like Collections) and new values appended

[remkop]

Currently array fields (Params or option) are always initialized to a new array instance, regardless of what the current value is.

Collections however are only instantiated when they are initially null, otherwise values are added to the original collection instance.

This asymmetry is undesirable.

Note: we may want to support map-like options like -Da=b -Dx=y. Behaviour for map-like containers should be consistent with arrays and collections.

[remkop]

Symmetry can be restored by treating arrays similar to collections: create a new array containing the old values as well as the new values.