// X-Requested-With header                // For cross-domain requests, seeing as conditions for a preflight are                // akin to a jigsaw puzzle, we simply never set it to be sure.                // (it can always be set on a per-request basis or even using ajaxSetup)                // For same-domain requests, won't change header if already provided.                if ( !options.crossDomain && !headers[ "X-Requested-With" ] ) {                    headers[ "X-Requested-With" ] = "XMLHttpRequest";                }請問這段代碼是什么意思呢？