val newKey = key?.lowercase(US)
      newAuthParams[newKey] = value
    }
    this.authParams = unmodifiableMap<String?, String>(newAuthParams)
  }

  /** Returns a copy of this charset that expects a credential encoded with [charset]. */
  fun withCharset(charset: Charset): Challenge {
    val authParams = this.authParams.toMutableMap()
    authParams["charset"] = charset.name()
    return Challenge(scheme, authParams)