easy:SetOpt_Chunk_End_Function(chunk_end_callback[, userdata])
The callback will not receive any parameters unless you pass the optional userdata
argument. In that case, the value you pass in userdata will be passed to your callback
function as a parameter. The userdata parameter can be of any type.
Return #CURL_CHUNK_END_FUNC_OK if everything is fine or
#CURL_CHUNK_END_FUNC_FAIL to tell the lib to stop if some error occurred.