Make Lua's wesnoth.transform_unit() function preserve hit points.

This is actually simpler to implement and is more consistent with the [transform_unit] tag.
Plus, the wiki documentation for wesnoth.transform_unit() does not state that the hit points
definitely will be changed (it's a bit vague on that point), so this is consistent with the
existing documentation.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56017 75b3775d-3801-0410-9e12-e2e43121521d