fix issue CLOUDSRV-2569 User Create generate 2 solr index calls in delay job queue.

in before save call back, tags always are assigned value, it is not
right, only it was changed, we need to reassign its value, otherwise in
some models, like Place, Event, User, tags is index fields, assign
value to tags will trigger an index call.
So, if no change for tags, no need to reassign its value.