code: function (field, value) {
          if (is(field).a(Object)) {
            for (var key in field) {
              if (Object.prototype.hasOwnProperty.call(field, key)) {
                $this.header(key, field[key])
              }
            }
  
            return $this
          }
  
          var existingHeaderName = $this.hasHeader(field)
          $this.options.headers[existingHeaderName || field] = value
  
          return $this
        }
docs: Sets header field to value
  
  @param  {String} field Header field
  @param  {String} value Header field value
  @return {Object}