`tagMap` is incorrectly cleared between reading documents, leading to exceptions

[hybridherbst]

Currently, state.tagMap is cleared on every document start:

js-yaml/lib/loader.js

Lines 1544 to 1554 in 0d3ca7a

	function readDocument(state) {
	var documentStart = state.position,
	_position,
	directiveName,
	directiveArgs,
	hasDirectives = false,
	ch;
	state.version = null;
	state.checkLineBreaks = state.legacy;
	state.tagMap = Object.create(null);

This is a bug – to the best of my understanding, this is valid YAML, where tags are declared at the start and then all the following documents can use those tags.

%YAML 1.1  
%TAG !u! tag:unity3d.com,2011:
--- !u!104 &render_settings # Document 1: Render Settings
RenderSettings:
  m_ObjectHideFlags: 0
  serializedVersion: 9
  m_Fog: 0
  m_FogColor: {r: 0.5, g: 0.5, b: 0.5, a: 1}
  m_AmbientSkyColor: {r: 0.212, g: 0.227, b: 0.259, a: 1}
--- !u!1 &game_object_1 # Document 2: A Game Object
GameObject:
  m_ObjectHideFlags: 0
  m_CorrespondingSourceObject: {fileID: 0}
  m_PrefabInstance: {fileID: 0}
  m_PrefabAsset: {fileID: 0}
  serializedVersion: 6

It will result in an error while the second document is parsed:

YAML Error: undeclared tag handle "!u!"