Added 'message id auto-generated' flag
Fixed default value of mMessageIDAutoGenerated
Fixed behavior of htmlForAbstractMultipartAlternative()
Merge pull request #818 from foxinushka/feature/message_id_autogenerated_flag

Added 'message id auto-generated' flag
Fixed #826: group all BODY[HEADER.FIELDS] (...)
Batch every 50 emails
Merge branch '826-headers'