feature/add-collection-item

added public function for adding new collection items; fixed bug when setting array collection;

- [ ] test coverage
- [x] finished the code
- [x] gathered feedback for my changes
- [ ] submited changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | [url to doc|none]