code: 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);
          }
docs: 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