What headers can be passed through in web gl

Right now it appears that the only headers that get passed through on a response from a WWW put request is CONTENT-TYPE and CACHE-CONTROL despite seeing in the web network tool of the browser (both chrome and firefox) that a lot more headers are being sent back. Is this a current limitation on what we are able to receive back, and if so is there another way that i can specify what headers i want to allow to be passed through?