ackFriendMessage(friendSteamId, timestamp) {
		this.user._sendUnified('FriendMessages.AckMessage#1', {
			steamid_partner: Helpers.steamID(friendSteamId).toString(),
			timestamp: convertDateToUnix(timestamp)
		});
	}
Acknowledge (mark as read) a friend message
@param {SteamID|string} friendSteamId - The SteamID of the friend whose message(s) you want to acknowledge
@param {Date|int} timestamp - The timestamp of the newest message you're acknowledging (will ack all older messages)