resp.Json()默认不传参，用法更符合requests

`func (resp *Response) Unmarshal(v interface{}) error {
	if resp.content == nil {
		resp.Content()
	}
	return json.Unmarshal(resp.content, v)
}

func (resp *Response) Json() (map[string]interface{}, error) {
	var result = make(map[string]interface{})
	if err := resp.Unmarshal(&result); err != nil {
		return nil, err
	}else {
		return result, nil
	}
}`