misc.merge: ensure that misc.none is never returned (#441)

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.

lua/cmp/utils/misc_spec.lua

@@ -33,5 +33,19 @@ describe('misc', function()
       },
     })
     assert.are.equal(merged.a, nil)
+
+    merged = misc.merge({
+      a = misc.none,
+    }, {
+      a = nil,
+    })
+    assert.are.equal(merged.a, nil)
+
+    merged = misc.merge({
+      a = nil,
+    }, {
+      a = misc.none,
+    })
+    assert.are.equal(merged.a, nil)
   end)
 end)