Fix applySnapshot to map<string,boolean> bug

[pioh]

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}}

[coveralls]

Coverage remained the same at 93.078% when pulling a12f0d6 on pioh:master into f773d22 on mobxjs:master.

[mattiamanzati]

Thanks a lot for this PR!
Can you please add on the tests folder in the map.ts file a test that reproduces this bug and now passes the test? (a valid test could be the one you linked https://mattiamanzati.github.io/mobx-state-tree-playground/build/index.html#src=import%20%7B%20types%2CgetSnapshot%2C%20applySnapshot%20%7D%20from%20%22mobx-state-tree%22%0Aimport%20%7B%20inspect%20%7D%20from%20%22mobx-state-tree-playground%22%0A%0Aconst%20Store%20%3D%20types.model('Store'%2C%20%7B%0A%20%20tags%3A%20types.map(types.boolean)%2C%0A%7D)%0A%0Alet%20store%20%3D%20Store.create(%7Btags%3A%20%7Badmin%3A%20true%7D%7D)%0Ainspect(store)%0A%0AapplySnapshot(store%2C%20%7Btags%3A%20%7Badmin%3A%20true%7D%7D)%0A%0AapplySnapshot(store%2C%20%7Btags%3A%20%7Badmin%3A%20true%7D%7D)%0A )

[mattiamanzati]

Please add a test for this case, so with futher changes we can be sure to not break the expected behaviour :)

[coveralls]

Coverage increased (+0.06%) to 93.141% when pulling 4bc510a on pioh:master into f773d22 on mobxjs:master.

[pioh]

I added one test. Thank you!

[mattiamanzati]

Thanks for your effort!