Fix nullref during CreateRoleAsync

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

In cases where Discord sent a value of `id=0`, this would throw an invalid-cast, where 0u64 cannot be cast to string.

this is all really broken

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

Fix #323

thanks to auxesis for the find

Moved IDeletable interface up inheritance to IMessage

Fix #320

I'm not entirely sure this is the proper way to fix this. However, it seems like
SendFileAsync returns the Task<RestUserMessage>, which would exit the using
block, disposing of the FileStream potentially before the Task is finished.

Changed to await the result of the task before exiting the using block.