fix: #874 setup return object with type of Module

[xialvjun]

isPlainObject is too strict for the returning value of setup function. Sometimes user may return { ...refs } in setup function, and Symbol(Symbol.toStringTag) in refs (eg: refs is a ESModule object), then { ...refs } will inherit that symbol property, then isPlainObject({ ...refs }) === false which may not that intuitive.

Besides, vue3 use isObject too. https://github.com/vuejs/vue-next/blob/87c86e4cc2/packages/runtime-core/src/component.ts#L688