Fixed a couple DiscordPresence properties returning null

Created by: Neuheit

Summary

This PR fixes #496 (closed).

Details

I discovered that that both DiscordPresence.Activities and DiscordPresence.GuildId were not initialized during the internal guild create handler, and were only set after a presence update. This PR should fix that.

Changes proposed

• Made both the DiscordPresence.Activities and DiscordPresence.GuildId initialize during the guild create handler.

Notes

On an unrelated note I found that the properties Activity and Activities somewhat confusing, and I feel that Activity should be renamed to CurrentActivity or something similar. Let me know your opinions on this.