Merge in small changes from our production environment

Allow for the User-Agent and Connection to be unset from the headers.

If the referrer is httprl_async_function_callback then use $_SERVER['HTTP_REFERER'] for the referrer; default to send referrer if using a httprl_queue_background_callback().

Fix for sending file/files in the data key.

ability for httprl_pr to output text vs html.

Fix httprl_call_user_func_array_cache to cut connection on shutdown function, fix potential loop as well.