GDScript : default value of an exported variable can be modified at run time

[ChickenStorm]

Godot version: v3.2.2.stable.official

OS/device including version: Windows 8.1

Issue description:
When using export var variable : Type = default_value, if default_value is a reference, it can be modified at run time. At the creation every variable will point to the same value. It means that one can modify variable of other instances of the class. It will also affect the value of variable at the creation of a new instance.

Steps to reproduce:

minimal code to reproduce

class_name CustomObject
extends Object

export var dict : Dictionary = {}

extends Node


func _ready():
	var a = CustomObject.new()
	a.dict["key"] = "test"
	a.free()
	var b = CustomObject.new()
	print(b.dict)
	b.free()

output : {key:test}

Minimal reproduction project:
minimal-project-export-default.zip

[akien-mga]

Related to #20436, might be duplicate of another issue but can't find it right now. CC @ThakeeNathees @vnen @bojidar-bg