append(name, value, isLowerCase) {
        this[kHeadersSortedMap] = null;
        const lowercaseName = isLowerCase ? name : name.toLowerCase();
        const exists = this[kHeadersMap].get(lowercaseName);
        if (exists) {
          const delimiter = lowercaseName === "cookie" ? "; " : ", ";
          this[kHeadersMap].set(lowercaseName, {
            name: exists.name,
            value: `${exists.value}${delimiter}${value}`
          });
        } else {
          this[kHeadersMap].set(lowercaseName, { name, value });
        }
        if (lowercaseName === "set-cookie") {
          (this.cookies ??= []).push(value);
        }
      }
@see https://fetch.spec.whatwg.org/#concept-header-list-append
@param {string} name
@param {string} value
@param {boolean} isLowerCase