Parameter preconditions were always getting the empty service provider,
even when a custom one was provided in ExecuteAsync, which means that
preconditions which use services cannot work properly.

💥

This was 👍'd in the dev chat, I forgot to make a PR for it (whoops!)

* Made GetOrCreateUser always call AddRef and added check to PRESENCE_UPDATE to avoid readding users who have been removed from guilds

* Removed AddRef as per dev guild discussion

* remove tip in docs about SocketEntity.Discord

* Expose the 'Fields' collection on EmbedBuilder

After some discussion I decided that there was really no reason to keep this private, and it didn't really go along with the rest of the design of the EmbedBuilder.

This is NOT a breaking change.

Exposing this property should not have any negative effects.

* Don't allow EmbedBuilder's Fields to be set to null

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

* Replace Where.FirstOrDefault with FirstOrDefault

* Replace Where+FirstOrDefault in ClientHelper

* Overloaded AddModuleAsync with Type

* Overloaded RemoveModuleAsync with Type

* Use expression-bodied method for consistency

The previous commit prevents any connections, since the initial presence update is sent while the client is still in the 'connecting' state, rather than the 'connected' state.

This resolves the original issue by preventing a nullref, and the more recent issue by only throwing a detailed exception when the CurrentUser is null (the client isn't logged in).

This prevents a later, less detailed nullref, when attempting to set the CurrentUser's presence data.

This also removes a redundant CurrentUser assignment in the SetGameAsync method, since this will be set later on in the SendStatusAsync method.

* Update ChannelHelper.cs

* typo

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