text: /**
   * Represents the Content-Type header.
   */
  export default class ContentType {
    /**
     * The type, subtype, and charset of the content type,
     * expressed in the HTTP header as follows:
     *
     *     [type]/[subtype]; charset=[charset]
     */
    constructor (type, subtype, charset) {
      this.type = type
      this.subtype = subtype
      this.charset = charset
    }
  
    toString () {
      const charset = this.charset == null
        ? ''
        : `; charset=${this.charset}`
  
      return `${this.type}/${this.subtype}${charset}`
    }
  }
  
  // Some predefined content types
  ContentType.JSON = new ContentType('application', 'json', 'utf-8')
  ContentType.TEXT = new ContentType('text', 'plain', 'utf-8')
  ContentType.HTML = new ContentType('text', 'html', 'utf-8')
  