add test case to break buggy code
bugfix related to AutoreleasePool's PoolManager

* duplicated AutoreleasePool added to PoolManager in
  PoolManager::getInstance()
* redundant pop_back() in PoolManager::~PoolManager()
remove unnecessary cache-variable
prefer std::vector to other containers
Merge pull request #6959 from kezhuw/bugfix_autorelease_pool_manager

Bugfix autorelease pool manager