Godot version:
3.1

OS/device including version:
MacOS, internal runing test

Issue description:
I save config to local with json and load it back next time. I found that the array object can't check unit with "find" or "has" api

Steps to reproduce:

1. read json file from user:// path and parse success.
2. get an array from dic which from jsondata.result
3. check if the array contain some data
4. result is always false or return -1
   ...
5. I then do manual loop check, the value can compare and can check it exist

Minimal reproduction project:
right code:

 if not dic.has(key):
	dic[key] = []
var arr:Array = dic[key] as Array
var isContain:=false
for k in arr:
	if k==value:
		isContain = true
		break
if not isContain:
	arr.append(value)
	saveData()

wrong code : (always save the same data)

 if not dic.has(key):
	dic[key] = []
var arr:Array = dic[key] as Array
var isContain:= arr.find(value)>=0 # arr.has(value)
if not isContain:
	arr.append(value)
	saveData()

dic defined:

var dic:Dictionary = {}

func _ready():
	saver.open("user://savegame.save", File.READ)
	var data = JSON.parse(saver.get_as_text())
	saver.close()
	if data!=null && data.result!=null:
		dic = data.result