'''Syntax:'''	'''proxy_cache_use_stale''' error | timeout | invalid_header | updating | http_500 | http_502 | http_503 | http_504 | http_404 | off ...
'''Default:'''	''off''
'''Context:'''	http server location
'''Reference:'''	[http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_use_stale proxy_cache_use_stale]