Enable groups_user_is_member() function with reference to PR

See itthinx/groups#59

modules/class-groups.php

@@ -228,6 +228,7 @@ public function modify_user( $user, $accessible ) {
 	/**
 	 * Filter the user-group relation.
 	 *
+	 * @todo https://github.com/itthinx/groups/pull/59
 	 * @see  Groups_User_Group::read() >> groups/lib/core/class-groups-user-group.php
 	 *
 	 * @since   1.7.x
@@ -237,15 +238,15 @@ public function modify_user( $user, $accessible ) {
 	 * @param   int   $group_id  Group ID.
 	 * @return  bool|object
 	 */
-	/*public function groups_user_is_member( $result, $user_id, $group_id ) {
+	public function groups_user_is_member( $result, $user_id, $group_id ) {
 		if ( (int) $user_id === (int) $this->store->get_curUser()->ID
 		     && $this->selectedGroup
 		     && (int) $group_id === (int) $this->selectedGroup->group->group_id
 		) {
 			$result = $this->selectedGroup->group;
 		}
 		return $result;
-	}*/
+	}
 
 	/**
 	 * Filter for the current view.