fix(observe): solve the Ref not unwrapping on the ssr side issue with recursive way.

[ygj6]

Solving #721 with #722：

  it('result of #722', () => {
    const state = { old: ref(false) }
    set(state, 'new', ref(true))
    expect(JSON.stringify(state)).toBe(
      '{"old":{"value":false},"new":true}'  // 'old' is different from 'new'
    )
  })

the form of value of old key is different from new key.

if we solve #721 with recursive way:

  it('solving with recursive way', () => {
    const state = { old: ref(false) }
    set(state, 'new', ref(true))
    expect(JSON.stringify(state)).toBe(
      '{"old":{"value":false},"new":{"value":true}}'
    )
  })

for above case, should {"old":{"value":false},"new":{"value":true}} is more appropriate? @pikax @antfu Look forward your comments？

[antfu]

Suggested change

	function walk(obj: any) {
	function walk(obj: any, seen = new WeakMap<any, boolean>()) {

Would need a map to avoid circular reference.