Json serializer writes `ObjectMap` incorrectly

[Blockzilla101]

Version: master-SNAPSHOT
Description: Trying write an ObjectMap with its key set to Team writes the team as a string instead of using its seralizer to write the team id.

Steps to reproduce: Run the following code:

public class Foo {
    public ObjectMap<Team, Integer> bar = new ObjectMap<>();
}

var foo = new Foo();
foo.bar.put(Team.sharded, 0);
JsonIO.read(Foo.class, JsonIO.write(foo));

foo looks like this after serializing: {bar:{sharded:0}}

Logs:

[10-02-2022 11:03:48] [E] arc.util.serialization.SerializationException: Serialization trace:
{}.bar
bar (bomberman.MapLoader$Foo)
	at arc.util.serialization.Json.readFields(Json.java:904)
	at arc.util.serialization.Json.readValue(Json.java:1077)
	at arc.util.serialization.Json.readValue(Json.java:980)
	at arc.util.serialization.Json.fromJson(Json.java:821)
	at mindustry.io.JsonIO.read(JsonIO.java:61)
	at bomberman.MapLoader.load(MapLoader.java:35)
	at plugin.BomberMan.lambda$reloadMaps$12(BomberMan.java:186)
	at mindustry.core.World.loadMap(World.java:340)
	at mindustry.core.World.loadMap(World.java:334)
	at mindustry.server.ServerControl.lambda$registerCommands$30(ServerControl.java:392)
	at arc.util.CommandHandler.lambda$register$2(CommandHandler.java:128)
	at arc.util.CommandHandler.handleMessage(CommandHandler.java:85)
	at arc.util.CommandHandler.handleMessage(CommandHandler.java:29)
	at mindustry.server.ServerControl.lambda$setup$6(ServerControl.java:156)
	at arc.util.TaskQueue.run(TaskQueue.java:17)
	at arc.backend.headless.HeadlessApplication.mainLoop(HeadlessApplication.java:81)
	at arc.backend.headless.HeadlessApplication$1.run(HeadlessApplication.java:52)
Caused by: java.lang.NumberFormatException: For input string: "sharded"
	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
	at java.base/java.lang.Integer.parseInt(Integer.java:668)
	at java.base/java.lang.Integer.parseInt(Integer.java:786)
	at arc.util.serialization.JsonValue.asInt(JsonValue.java:261)
	at mindustry.io.JsonIO$8.read(JsonIO.java:172)
	at mindustry.io.JsonIO$8.read(JsonIO.java:164)
	at arc.util.serialization.Json.readValue(Json.java:1084)
	at arc.util.serialization.Json.readValue(Json.java:980)
	at arc.util.serialization.Json.readValue(Json.java:1034)
	at arc.util.serialization.Json.readFields(Json.java:897)
	... 16 more

[Anuken]

JSON keys must always be strings. Numbers cannot be keys.