语法: value = ngx.headers_sent
环境: set_by_lua, rewrite_by_lua*, access_by_lua*, content_by_lua**
如果应答头部已经被发送（通过 ngx_lua）返回 true ，否则返回 false 。
该 API 是在 v0.3.1rc6 版本首次引入的。
English source:
syntax: value = ngx.headers_sent
context: set_by_lua, rewrite_by_lua*, access_by_lua*, content_by_lua**
Returns true if the response headers have been sent (by ngx_lua), and false otherwise.
This API was first introduced in ngx_lua v0.3.1rc6.