Update test to match new behavior for arrays and update readme

Author: iwiznia

README.md

@@ -63,7 +63,7 @@ The data will then be cached and stored via [`AsyncStorage`](https://github.com/
 
 We can also use `Onyx.merge()` to merge new `Object` or `Array` data in with existing data.
 
-For `Array` the default behavior is to concatenate new items.
+For `Array` the default behavior is to concatenate replace it fully, effectively making it equivalent to set:
 
 ```javascript
 Onyx.merge(ONYXKEYS.EMPLOYEE_LIST, ['Joe']); // -> ['Joe']
@@ -77,7 +77,7 @@ Onyx.merge(ONYXKEYS.POLICY, {id: 1}); // -> {id: 1}
 Onyx.merge(ONYXKEYS.POLICY, {name: 'My Workspace'}); // -> {id: 1, name: 'My Workspace'}
 ```
 
-Arrays inside objects will NOT be concatenated and instead will be replaced fully:
+Arrays inside objects will NOT be concatenated and instead will be replaced fully, same as arrays not inside objects:
 
 ```javascript
 Onyx.merge(ONYXKEYS.POLICY, {employeeList: ['Joe', 'Jack']}); // -> {employeeList: ['Joe', 'Jack']}

tests/unit/onyxCacheTest.js

@@ -267,9 +267,9 @@ describe('Onyx', () => {
                     mockKey: [{ID: 3}, {added: 'field'}, {}, {ID: 1000}],
                 });
 
-                // THEN the arrays should be merged as expected
+                // THEN the arrays should be replaced as expected
                 expect(cache.getValue('mockKey')).toEqual([
-                    {ID: 3}, {ID: 2, added: 'field'}, {ID: 3}, {ID: 1000},
+                    {ID: 3}, {added: 'field'}, {}, {ID: 1000},
                 ]);
             });