feat(Emoji): add fetchAuthor method

Backported from commit: e0cbf0bb60
PR: #2315

src/structures/Emoji.js

@@ -150,6 +150,15 @@ class Emoji {
     return this.edit({ name }, reason);
   }
 
+  /**
+   * Fetches the author for this emoji
+   * @returns {Promise<User>}
+   */
+  fetchAuthor() {
+    return this.client.rest.makeRequest('get', Constants.Endpoints.Guild(this.guild).Emoji(this.id), true)
+      .then(emoji => this.client.dataManager.newUser(emoji.user));
+  }
+
   /**
    * Add a role to the list of roles that can use this emoji.
    * @param {Role} role The role to add