contains (name, isLowerCase) {
    // A header list list contains a header name name if list
    // contains a header whose name is a byte-case-insensitive
    // match for name.

    return this.headersMap.has(isLowerCase ? name : name.toLowerCase())
  }
@see https://fetch.spec.whatwg.org/#header-list-contains
@param {string} name
@param {boolean} isLowerCase