Allowed schemas in a node list field can't be changed after saving

## Gentics Mesh Version, operating system, or hardware.
- v1.3.2

## Operating System
- MacOSX

## JVM
- Oracle JDK 1.8.0_202

## Problem
After saving them once, allowed schemas in a node list field can not be changed anymore in the UI.

## Reproducer
* Start a fresh instance of mesh: `MESH_INITIAL_ADMIN_PASSWORD_FORCE_RESET=0MESH_INITIAL_ADMIN_PASSWORD=admin java -jar mesh-server-1.3.2.jar`
* Go to the admin area and create a new Schema named "test"
* Add a field named "test_field" of the type "list", the list type "node" and add the "Allowed Schema" "binary_content"
* Save it
* Try to remove or add "Allowed Schemas", it won't work
* Switch to the "JSON" tab, try to add or remove elements in "allow". If you save it and reload the page, your changes will not appear.

## Expected behaviour and actual behaviour
The "Allowed Schemas" should be changeable after saving them once.

Start in the admin area schema editor and reproduce the issue by creating a list node field with an allowed schema, saving it, then changing the allowed schemas. Check both the regular editor and the JSON tab; done means additions and removals persist after saving and reloading.

Written by the indexing model from the issue text.