Before this change:
1. `misc.merge({a = misc.none}, {a = nil})` returned `{a = misc.none}`
2. `misc.merge({a = nil}, {a = misc.none})` returned `{a = misc.none}`
(1) can cause error if a non-existing mapping is set to `config.disable`
(which is an alias for `misc.none`).
(2) does not cause any issue to date, but is inconsistent with the
expected behavior of `misc.merge(…)`.
After this change:
1. `misc.merge({a = misc.none}, {a = nil})` returns `{a = nil}`
2. `misc.merge({a = nil}, {a = misc.none})` returns `{a = nil}`
Fixes#440.
