[mbox_parser] Improve message Subject normalization

[lukas-vlcek]

See:
http://lists.jboss.org/pipermail/rules-users/2011-May/020329.html
http://lists.jboss.org/pipermail/rules-users/2011-May/020328.html

They contain subjects with "[Drools Flow 5.1]" in it, however, it is removed by mornalization logic. Should this be kept in Subject?