-type="application/json"`
       *      * body is encoded like `{"foo":"bar baz","x":1}`
       *    * `content-type="application/x-www-form-urlencoded"`
       *      * body is encoded like `foo=bar+baz&x=1`
       */
      contentType: {
        type: String,
        value: "application/x-www-form-urlencoded"
      },

      /**
      * HTTP request headers to send.
      *
      * Note: setting a `Content-Type` header here will override the value
      * specified by the `contentType` property of this el