Allow presence updating to allow null games (#1186)

* Adding resetGame functionallity

`setGame` method would allways result in an Object passed to `setPresence`.
Passing { game: null } (supported by discords WebSocket gateway to reset the current Game) to `setPresence` would still result in a Game Object sent to the endpoint.
Explicitly setting `game` to null should overwrite the `game` object provided by `localPresence` or `client.presence`.
This was neither supported by `setGame` or `setPresence`.

* Missing semicolons to resetGame and setPresence

* Fixing trailing spaces, commas and semicolons

* Moving resetGame functionality into setGame method

Minification of if statement in setPresence.
Removing resetGame method and adding a case for `game === null` to setGame method

* Adding missing space in setGame method

* Fix docs

src/structures/ClientUser.js

@@ -186,6 +186,8 @@ class ClientUser extends User {
         if (game.url) game.type = 1;
       }
 
+      if (data.game === null) game = null;
+
       if (typeof data.afk !== 'undefined') afk = data.afk;
       afk = Boolean(afk);
 
@@ -224,11 +226,12 @@ class ClientUser extends User {
 
   /**
    * Sets the game the client user is playing.
-   * @param {string} game Game being played
+   * @param {?string} game Game being played
    * @param {string} [streamingURL] Twitch stream URL
    * @returns {Promise<ClientUser>}
    */
   setGame(game, streamingURL) {
+    if (game === null) return this.setPresence({ game });
     return this.setPresence({ game: {
       name: game,
       url: streamingURL,