Flush buffer for groups after add/remove operations in Eloquent\User mod...

...el

We should flush cached `$this->userGroups` value after `attach()`/`detach()` group to/from user. Second solution for this is to append/delete group to/from buffer, but for appending case we don't know order into collection (it may be override in the `groups()` method). So it is a simplest implementation.