Corrected bug in triggering of Schedules 

This is a possible fix for issue https://github.com/michaelklishin/quartz-mongodb/issues/68

This correction will 
1. Fix return of "null" in getScheduledFireTime() for the first time.
2. Make sure that the scheduledFireTime is correct for all triggers.
[RAMJobStore](http://svn.terracotta.org/svn/quartz/tags/quartz-2.1.7/quartz/src/main/java/org/quartz/simpl/RAMJobStore.java) does this.  But, we miss to do this.