Added forgotten new comment validation in PostController.
Updated Comment class hashCode() method to allow work with nullable values.
Added NotEmpty validation annotation to Post's field 'test'
Added checking tags for null in Post's getTagsString method.
Removed cascade operations from Tag's field 'posts'.
Changed TagDao contract - added deleteTag(Tag) method.
Updated TagDaoImpl according to new version of interface.
Removed outdated comment from MessageController.
Updated processTags method of PostController to delete unnecessary tags.
Updated addPost method of PostController to validate post before saving
Updated updatePost method of PostController to validate post before saving