get (name, isLowerCase) {
    // 1. If list does not contain name, then return null.
    // 2. Return the values of all headers in list whose name
    //    is a byte-case-insensitive match for name,
    //    separated from each other by 0x2C 0x20, in order.
    return this.headersMap.get(isLowerCase ? name : name.toLowerCase())?.value ?? null
  }
@see https://fetch.spec.whatwg.org/#concept-header-list-get
@param {string} name
@param {boolean} isLowerCase
@returns {string | null}