lib/request.js
function(field){
            var req = this.req;
            field = field.toLowerCase();
            switch (field) {
                case 'referer':
                case 'referrer':
                return req.headers.referrer || req.headers.referer || '';
                default:
                return req.headers[field] || '';
            }
        }
function(field){
            var req = this.req;
            field = field.toLowerCase();
            switch (field) {
                case 'referer':
                case 'referrer':
                return req.headers.referrer || req.headers.referer || '';
                default:
                return req.headers[field] || '';
            }
        }
function ( field ) { var req = this . req ; field = field . toLowerCase ( ) ; switch ( field ) { case 'referer' : case 'referrer' : return req . headers . referrer || req . headers . referer || '' ; default : return req . headers [ field ] || '' ; } }
Return request header.

The `Referrer` header field is special-cased,
both `Referrer` and `Referer` are interchangeable.

Examples:

this.get('Content-Type');
// => "text/plain"

this.get('content-type');
// => "text/plain"

this.get('Something');
// => undefined

@param {String} field
@return {String}
@api public
Return request header .