function (field, value) {
    if (arguments.length === 2)
      this.headers[field] = value;
    else {
      for (var prop in field)
        this.headers[prop] = field[prop];
    }
    return this;
  }
function (field, value) {
    if (arguments.length === 2)
      this.headers[field] = value;
    else {
      for (var prop in field)
        this.headers[prop] = field[prop];
    }
    return this;
  }
function ( field , value ) { if ( arguments . length === 2 ) this . headers [ field ] = value ; else { for ( var prop in field ) this . headers [ prop ] = field [ prop ] ; } return this ; }
Sets `field` header to `value`, or accepts an object and applies
those key value pairs to the Response object's headers.

@param {String|Object} field
@param {String} value
@return {Response}
Sets field header to value or accepts an object and applies those key value pairs to the Response object s headers .