Fix applySnapshot to map<string,boolean> bug
Fix applySnapshot to map<string,boolean> bug

Primitive types removes from the map on applySnapshot if key exists.

const Store = types.model('Store', {
  tags: types.map(types.boolean),
})
let store = Store.create({tags: {admin: true}})
console.log(JSON.stringify(getSnapshot(store)))
applySnapshot(store, {tags: {admin: true}})
console.log(JSON.stringify(getSnapshot(store)))
applySnapshot(store, {tags: {admin: true}})
console.log(JSON.stringify(getSnapshot(store)))

Output:
{"tags":{"admin":true}}
{"tags":{}}
{"tags":{"admin":true}}
mobxjs/mobx-state-tree,mobxjs/mobx-state-tree,mweststrate/mobx-state-tree,mweststrate/mobx-state-tree,mobxjs/mobx-state-tree,mobxjs/mobx-state-tree