Options constructor
      
      Options({ 
    
    
- String? method,
- Duration? sendTimeout,
- Duration? receiveTimeout,
- Map<String, dynamic> ? extra,
- Map<String, dynamic> ? headers,
- bool? preserveHeaderCase,
- ResponseType? responseType,
- String? contentType,
- ValidateStatus? validateStatus,
- bool? receiveDataWhenStatusError,
- bool? followRedirects,
- int? maxRedirects,
- bool? persistentConnection,
- RequestEncoder? requestEncoder,
- ResponseDecoder? responseDecoder,
- ListFormat? listFormat,
Implementation
Options({
  this.method,
  Duration? sendTimeout,
  Duration? receiveTimeout,
  this.extra,
  this.headers,
  this.preserveHeaderCase,
  this.responseType,
  this.contentType,
  this.validateStatus,
  this.receiveDataWhenStatusError,
  this.followRedirects,
  this.maxRedirects,
  this.persistentConnection,
  this.requestEncoder,
  this.responseDecoder,
  this.listFormat,
})  : assert(receiveTimeout == null || !receiveTimeout.isNegative),
      _receiveTimeout = receiveTimeout,
      assert(sendTimeout == null || !sendTimeout.isNegative),
      _sendTimeout = sendTimeout;