Adds the `headers`-object to `xhr`-headers.

@method _setHeaders
@param xhr {Object} containing the xhr-instance
@param headers {Object} containing all headers
@param method {String} the request-method used
@private
function(xhr, promise, headers, method) {
            // XDR cannot set requestheaders, only XHR:
            if (!xhr._isXDR) {
                var name;
                if ((method!=="POST") && (method!=="PUT")) {
                    // force GET-request to make a request instead of using cache (like IE does):
                    headers["If-Modified-Since"] = "Wed, 15 Nov 1995 01:00:00 GMT";
                    // header "Content-Type" should only be set with POST or PUT requests:
                    delete headers[CONTENT_TYPE];
                }
                // set all headers
                for (name in headers) {
                    xhr.setRequestHeader(name, headers[name]);
                }

                // in case of POST or PUT method: always make sure "Content-Type" is specified
                !BODY_METHODS[method] || (headers && (CONTENT_TYPE in headers)) || xhr.setRequestHeader(CONTENT_TYPE, DEF_CONTENT_TYPE_POST);
            }
        }