Fix nested object spread (#4974)

packages/babel-plugin-transform-object-rest-spread/test/fixtures/object-spread/assignment/actual.js

@@ -1 +1,3 @@
 z = { x, ...y };
+
+z = { x, w: { ...y } };

packages/babel-plugin-transform-object-rest-spread/test/fixtures/object-spread/assignment/expected.js

@@ -1,3 +1,5 @@
 var _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; };
 
 z = _extends({ x }, y);
+
+z = _extends({ x, w: { ...y } });