Undocumented way to remove metadata variants in Object

[Rubonnek]

Your Godot version:
3.2

Issue description:
The Object class documentation does not state that using null in a set_meta call deletes the specified metadata entry.

#!/usr/bin/godot -s
extends SceneTree

func _init():
	print("Unmodified Meta List")
	print(get_meta_list())
	print("New test meta")
	set_meta('test', true)
	print("Modified Meta List -- added: test")
	print(get_meta_list())
	set_meta('test', null)
	print("Modified Meta List -- removed: test")
	print(get_meta_list())
	quit()

This should be documented.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/classes/class_object.html