Doc update for modify user sample code

Make header dictionary ignore case

Resolves an issue where the ratelimiter would panic.

Fix nullref during CreateRoleAsync

Access "Date" in response header safely with TryGetValue. Hopefully nothing bad happens when lag is 0

Property can be renamed as needed.

Update ReplyAsync Task to return the sent message.

This property previously had a default value of 'null'. In DiscordSocketClient#ConnectInternalAsync, we would attempt to send a null status to Discord, which would fail to serialize, and force the client to disconnect.

Adjusted GetGuildPruneCount to using params;

fixes #240