The first parameter to Object.assign() is the same target object that
will be returned. That is, since commit 46874eed0 Params.parse() modifies
the @defaults object. Usually we pass that parameter as an object literal
and this isn't an issue, but the change breaks spectacularly in the few
cases where we use a re-usable variable.
Restore the previous behavior by copying the object first.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/615
8a5de327bb