Steps to reproduce:

1. create room (at this time user receives message with empty <subject/> after initial presence)
2. configure the room and set it as persistent
3. leave and re-join the room - there is no <message/> packet with <subject/> being sent.

Initial analysis: While this issue was reported earlier (#issue #103) and fixed in commit:c7b63584 the problem exists and is most likely a result of repository rework (switch from UserRepository to dedicated MUC repository). Previously in MUCDao we were also calling setSubject(); when creating the room. Currently call to setSubject(); doesn't happen without actually changing the room, and without it subject_creator_nick and subject_date are not populated in the database when persisting the room.

Proposed solution(s):

1. make sure that subject details (creator, date) are stored in the database when persisting the room
2. correct tigase.muc.modules.PresenceModuleImpl#processEntering method responsible for generating the message (around L617) so in case of missing subject/subjectCreator/SubjectDate it would simply generate stanza with empty subject and would use room.getCreationDate() as a fallback.