lib/response.js
function(field, val){
            var prev = this.get(field);

            if (prev) {
                val = Array.isArray(prev) ? prev.concat(val) : [prev].concat(val);
            }

            return this.set(field, val);
        }
function(field, val){
            var prev = this.get(field);

            if (prev) {
                val = Array.isArray(prev) ? prev.concat(val) : [prev].concat(val);
            }

            return this.set(field, val);
        }
function ( field , val ) { var prev = this . get ( field ) ; if ( prev ) { val = Array . isArray ( prev ) ? prev . concat ( val ) : [ prev ] . concat ( val ) ; } return this . set ( field , val ) ; }
Append additional header `field` with value `val`.

Examples:

this.append('Link', ['<http://localhost/>', '<http://localhost:3000/>']);
this.append('Set-Cookie', 'foo=bar; Path=/; HttpOnly');
this.append('Warning', '199 Miscellaneous warning');

@param {String} field
@param {String|Array} val
@api public
Append additional header field with value val .